In buzztracker you can load one song over the top of another one and it would merge them together. I would be good if renoise had the ability to import another song and place them in a set of tracks next to the current ones. This feature is useful because sometimes half-finsihed songs could be merged together, fixed up a bit and become a new song. Also, sometimes you just load up a synth and play around with stuff and come up with some good ideas. It would be good if you could merge a bunch of these together.