I don't see why you would have an issue soldering bare copper to tinned wire. I don't know what type the stock stator wire is, but I would assume it is tinned copper. You can tin the bare copper wire you are adding by stripping the end, heating well and applying an even coat of solder. Tinning the wire will allow it to take solder easier and faster. You may also want to use a little liquid flux on your connection to aid in solder flow.
Make sure you cut your stator wires back into fresh strands, same thing with the wire you are adding.
If its not happening, go get some heatshrink butt connectors, they do an excellent job and require no soldering.