[Geoff Bacon]

@Steven

You might already be doing this so apologies if so.

Put all your separated songs into it's own setlist
Choose a common set of characters at the start of each song name (so that they are obviously a block of songs)

When needing to create a new setlist which includes them:-

1. Enter all other songs, if any, before the required posistion.
   
2. Exit the setlist editor.
   
3. Long Press this special song list until it has a check mark
   
4. ... (three dots, top right);  Select "add songs to a setlist"  (add to the one you were creating)
   
5. Re-edit the setlist;  all the songs will be co-located at the bottom (with the same leading prefix to show they are to be treated as a block).
   
6. Add any other songs.
   

Not brilliant but workable.
Could be tricky if you want to shuffle "blocks of songs" around the combined setlist.

Geoff