It turns out that it's not as hard as I thought to add melodies to the "beat/bassline" editor and song editor

As it turns out, you can make a full rhythm in the step editor, then clone it into different variations.
Along the way, you can add pitched instruments and then tweak them in the pianoroll. I found out that a pattern could just have one melodic part in it within the song editor. And multitple patterns can play simultaneously in the song editor.
So ultimately, you can create full tune by pseudo"overdubbing" melodic parts to go with the drum chains.
I'm happy that it's versatile this way. After I get more ideas down, then I will export the stems into REAPER.

