Like the title says, the shift cable some some routing through the bike frame so I don't want to remove it. I think to remove it would also require me to remove the mid drive motor.

What's the best way to extend or add on to it? I'm thinking just cut a piece of shift cable and use electrical shrink wrap to hold it.

You should try that and let us know how well it works.

I think this may be a better option:

Cable splitters for S&S Coupled travel bikes

You can probably find other options as well with a two minute Google (or your favorite search engine) search.

I have never had great success splicing a bicycle cable to lengthen it. I have tried though. It always looked super BAD. Just get yourself another cable...

Youtube: Incredible Huge Rope Splicing & Fabricating Process - Amazing Factory Machines Production Technology

some internal cables are easier to route than you think, just make sure your string is good quality and securely attached to the cable you're removing.

If you do end up needing to buy another cable/housing, get one for a tandem. It will have the added length you need.

Post pictures of what you are trying to achieve here.

One tip I discovered here on BF is to use the old cable as a fish wire, cutting any ends off the old cable and connecting the old and new cable with a length of thin (such as 1/16" I.D.) shrink tube long enough for good grip as it's pulled through. Worked great for me on my most recent attempt.

Sorry, I didn't mean cable. Of course you can't extend a cable. I meant cable housing.

You can get housing end caps that are one unit, but open at both ends with a separator in the middle of the cap, ergo, you need two pieces of housing of the correct lengths. Or, you can use two housing end caps with the closed end back to back, and the same with two pieces of housing. The cable runs straight through either option. With the back to back caps, you could wrap them with tape of the same color, but it is not necessary. Either way, can be seen only from close up. They work just like the cable stops on the frame.

Using just tape to splice the housing together will work, but not work that well. the connection just is not stiff/strong enough, IME.

If you are not fussy and want to use one of the more expensive housing brands like Jagwire or Shimano, standard cable housing is pretty inexpensive bought online. I'm still working with a 25 foot roll of shifter cable purchased from Nashbar more than a decade ago. This happens to be a source located one town away who sells on eBay.
12 foot roll of 5mm shift cable $16 postpaid https://www.ebay.com/itm/291917358195 same company 25 foot roll $25 postpaid https://www.ebay.com/itm/391259402350

Sheldon Brown (or John Allen) says splicing a cable is easy and does not require any extra hardware:

https://www.sheldonbrown.com/heroic-repairs.html

scroll down to the "splicing cables" section

I've used shrink tubing for this purpose (over the back to back ferrules) with good results.

Actually, you CAN extend cable (post #3), and much easier than extending cable housing.

Why does it have to be headless?...

Do you have a little metal (pref aluminum) cylinder? Could drill and tap a hole on either end of it and use socket head set screws to grab the cable. 4-40 screw should be plenty strong. Simple, strong, and elegant, but you’d need the tools

A quicker way could be just tie a knot. I doubt it would work but you could try, it’s probably what I would do just to see if it works.

I agree rerouting it seems like a *****

another option could be to get a nice zip tie, and use a lighter and a needle to poke two holes through it, tie the zip tie to itself. Thread the housing through the melted holes and then figure out how to make the end bunched up so it can’t fit back through the melted hole in the zip tie. One looped knot and some glue so the loop doesn’t slip out should work fine. The zip tie is acting like a shackle, it would probably hold honestly.

lol Sheldon just uses a square knot?

Good Lord, 20 posts. My newest bike has internally routed cable but I haven't had to replace it. I didn't realize it was this hard. Maybe it is, maybe it isn't, but I have to think Park Tool or Sheldon Brown or some other technical authority has a best practice tutorial in one of the usual places. If I had to do it myself I would already be on it. Let's get this thread notched up to #useful status and find a peer reviewed authority to post a link from.

Edit: A start? <shrug>

Cool internally routed cables, Sheldon Brown was posted and it looks like he just tied another cable to it to extend it lol. Its what I would have done but Im surprised it works

I have replaced internally routed cables/housing once on my Orbea Avant. I bought the Shimano routing tool, but, after doing the work, I realized I could have done it without the tool with no more time and hassle involved. I dreaded doing it the first time, but I am pretty sure if/when I do it again, it will go a lot quicker. I believe different manufactures use different techniques in the routing, ergo, there will be some differences in the replacement process. It seems likely to me that frame sets with oversized tubing make it a bit easier.

I use the cable as a guide, sliding the housing out of the frame leaving the shift/brake cable intact then sliding the new housing up the cable. I don't worry about the length at that moment, only that it extends beyond both ends of the frame, then I remove the cable and size the housing cutting it to the correct length. I then install a new cable and voila I'm done.

Yup, that's how I usually do it, also teflon tubing worlks well, some frames come with some on them, and I have a bunch from racquet stringing. Most newer frames aren't the routing nightmare that some older ones were, but every so often you get that one that wants to test you.

I would use one of these:

https:/jagwire.com/products/small-parts/connecting-junction-ferrules

Don’t even need to tape it if you don’t want to. Had one of these on hand when running the cables for my commuter. I did not have any housing runs long enough for what I needed and this worked great.

Brilliant, I did not know this existed; I was going to recommend a JagWire barrel adjuster for shift cable housing.