[Geoff Bacon]

@Beemer

I don't think you can do what you are trying to do (I could be wrong!)

I suggest you just create another setlist for your performance (does anyone do these in the current crisis?) 
All your single sheet songs can go directly into this setlist (called "Gig" say); a song can be in more than one setlist.
For your song books, you need to create entries equivalent to those you had imported as single songs. You can do this by creating a snippet (press centre of screen and press the "filled triangle in a circle" (bottom left). You can then specify the name of the song and the page number(s) it occupies.
Having created this entry, you can add it to your "Gig" setlist.

If your songbooks contain many songs, the best way is to create a .csv file that contains entries for each song. Then remove the current songbook and re-import it together with its .csv file.
The result will be that you have a single physical file for the songbook but it will look as though you imported individual song files.
The format of the .csv file needs to be just right (search forum for other posts) so it is a good idea to get it working with just 2 entries (say). That way it is relatively easy to delete the bad entries and retry using a corrected .csv

Cheers
Geoff