Actually, you can do this, but you have to do it in a rather haphazard way.
You can use 'midi-loops', which can be saved from your session as presets and imported via the media bay.
Midi loops are the same as midi parts, but with the above mentioned benefit of showing up in the media bay.
The unfortunate design flaw is that you can only export midi-loops from instrument tracks (don't ask me why).
So what i tend to do (as I rarely if ever use instrument tracks) is create an empty one with no vsti assigned, and copy-drag any midi part I want to save for later onto there; then it can be exported via the file->export->midiloop command.
If I want to reuse it, I simply browse for midi-loops only in the media bay and can then re-import them.
My main use for it is to save sketches....
Hope that helps,