I don't understand what the problem is... you can copy from any track in any song to any other track in any other song, thus have 160 tracks available for preparation of the final result from one load, not counting single song save and load to give unlimited flexibility. So you can prepare different tracks in different songs and then copy them across to combine them in one song.
Not being clear about exactly how you wish to proceed, it would surely be much easier to use the composer as a drum pattern creator, string the different sections together by real or much easier chord step record, and then produce a final track with apc to smf convert. This track could be as many tries as you like, and copied to as many songs as you like to monitor a final mix.
Just because the composer has bass and accomp does not matter - just ignore them and use the drum track. You have 36 programmable patterns of 16 measures each without counting the intros and endings in a composer load. This should be enough for the number of drum pattern variations you need for a single song creation. If you need more, step record from the custom (an additional 240 16 measure programmable patterns available, intro and ending extra).
apc conversion gives you a midi format type drum track, then you can play real time bass and accomp on top to produce a final result. The composer is useful to give a temporary scratchpad type mix for you to perfect the drums, this can then be discarded for a professional result if playing the rest of the tracks in real time multitrack.
It all depends on which is your personal easiest way of working really.