I’ve always thought it would be really nice to be able to do that. You can do it in Ableton… drag tracks or an entire song from one and include it in another. It’s a great way to build new ideas quickly - get the drum track from one song, bass from another, atmosphere from another…
I’d love it if renoise could do this. I saw some tool recently that is supposed to let you do at least a part of it, but I haven’t checked it out.
I do what you do and just spend some time making instruments w/ phrases which I then load into my projects.