Sure you can do it.
Those riffs are just MIDI loops. Drag a pattern(clip) to your MIDI track, as you would to include it in a song.
Then open the Piano Roll View. Here you can modify the clip, or add your own notes.
Or you can use a MIDI controller to play your own notes.
It all happens in the MIDI track associated with the synth. All of your soft synths are driven by the MIDI track, so once you get this idea down, you are pretty much home free.