I want to add drops to my R20, and I have a Sachs 3x7 rear hub. Will SunTour bar-end shifters work, I have a flat bar with grip shifters now and it works, but was trying to see if drops would gives me more comfort and possibily more speed.
I don't have experience with those specific components, but from what I know of IGH's you will not be able to use a bar-end shifter for the hub part of your 3X7 unless it is indexed specifically for a Sachs 3-speed hub, and I don't know if there is such a thing. The 7 cogs will not be a problem, I expect, as long as your shifters are friction or are indexed to the gear cluster. But you cannot friction shift a 3-speed hub. Or, rather, you can, but it's better not to: better for your hub and better for ease of use.
Also, I beleive Sram hubs maintain the same shifting as they did when they were Sachs, so you may get away with a Sram shifter. I would assume, but could easily be wrong, that the indexing for the hub portion of a 3X7 is exactly the same the indexing for a standard 3-speed hub.
If the suntour bar end shifter are shimano STI compactable it should work ok. However you will need a way of holding the rear cable in place to the internal and holding the outer cable still as the inner moves through it. You can just the clip that I managed some from an old LBS. I have converted 3 folders like this. 2 mezzos(3X9sram) and one space genie(3X7sach). Cable retainer clip visable on rear chain stay,You can also DIY something I guess