add rear derailleur
 

This is backwards for this group but I have a 16" tired folder -single gear. I have a used 8sp cassette, shifters, cables, derailleur etc from a 26" tired mountain bike. How hard (impossible?) would it be to put the gears on the folder? It has a rear band brake which might add complications. I do my own repairs but nothing like this before. thanks

what's the rear spacing? does it have a derailleur hanger?

p.s. the bike mechanics forum can probably help more

if the folder is a single gear it probably won't have a cassette-type hub. you could potentially use a thread-on freewheel if you can find one in 8sp, or something like a 5sp one with friction shifters. of course that supposes that you can fit a derailer, shifters, and cables on the frame without compromising their usability, since it likely wasn't designed for it.

if the rear spacing is 120mm you can use a 5 or 6sp freewheel..
8/9/10sp hubs require 130mm..
you will need another wheel.
band brake will probably have to go

maybe re-dishing the wheel will also have to be done? not sure if small folder wheels require it.