Hello,
Does anyone know if any frame builders would be willing to add disc brake mounts to a frame / fork???

This is perhaps one of the most common requests and the answer always depends on what frame and fork. It would be helpful to provide pictures of the frame and fork and also to tell us where you are located.

If it's a steel frame and you are happy with a disc mount on the back of the seat stay then it's likely that a disc mount can be added to the frame. It also needs a brace between the seat stay and the chain stay. The fork is a different matter. If it's a relatively heavy unicrown fork, then it probably would be okay. Might be better to buy a disc fork

If you are willing to spend some money and can find someone to do it you can cut the old dropout off and use some of the larger dropout assys from Paragon Machine to replace the cut portion of the SS and CS

https://www.paragonmachineworks.com/

https://www.paragonmachineworks.com/...12-mm-hub.html

https://www.paragonmachineworks.com/...ets-12-mm.html

https://www.paragonmachineworks.com/...omponents.html

Bike Fab Supply also has some dropouts that would work

https://www.bikefabsupply.com/rear-dropouts-thru-axle

I am located in Oceanside, California.
1st bicycle is Centurion LeMans
2nd is Specialized Hardrock &
3rd is Schwinn Skyliner
I can add pictures later

I wouldn't do this to the Lemans, the fork can't take it for one thing. The Hardrock is probably okay, and it depends on the vintage of the Skyliner

As Unterhausen said, the LeMans fork can't take the rotational force of the disc brake. The frame probably doesn't have clearance for the brake rotor at the seat stay or chainstay. The Hardrock is Aluminum, so you probably won't find anyone willing to do that job because it needs heat treating after welding. The Skyliner also appears to be aluminum and my Google search shows the current model to be a $200 department store bike that wouldn't really benefit from the upgrade to disc brakes.

thanks for your answers!!