Hopefully this works, I do this on my SEQUENCER, but not with SMF, but on my original sequenced tunes if I need to add measures later. Results may vary...

Once file is loaded in Sequencer

1. MENU
2. SONG EDIT
3. TRACK -Checkmark ALL TRACKS cause you want to insert 8-16 bars for whole song
4. CUT INSERT (You may have to use COPY function instead here since you have pre-existing music you need still playing while you solo? same theory copy measures and insert ALL TRACKS)
5. INSERT MEASURES where you want
6. EXECUTE
7. SAVE YOUR NEW SEQUENCE

****SOMETIMES THIS AND FORMER PA 4X will not highlight the EXECUTE...It is a SOFTWARE glitch and make sure your song is saved, cause you will need to reboot. The EXECUTE function will work when rebooted...

Surprised Korg doesn't know about this issue...It is a hassle when I am sequencing...Have good back up habits

Not sure if this helps or what you needed...But I thought that is what you need...

Let me know if this at least gets you closer to what you are trying to accomplish