As the cables wont attach to each other, butt a GT transformer up against your IC2 stash (Batbox etc). Should convert fine, not tried it with the bigger supplies but def works fine with a batbox (Doing it in my current GT world 'cause the lack of burnerating generators in the new GT makes me a sad panda).
Trick is to make sure you don't try wiring the transformer to the input, it won't work. Just make sure the dots all line up like you normally would and it should work
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Think this only works IC2->GT, not sure about the reverse.