I don't see any reason why it would be a problem. The only obstacle is that you'd have to provide .upp file. You could do this in theide or manually. Just create main package containing all the non-U++ code and make it depend on the U++ packages it uses. Both umk and theide don't really care if the code is U++ or not.
You are right. I will generate the .upp file automatically and call umk with it. Thank you .