This is simple, just had my buddy going through the same thing.
He was using a claw sling, but most slings are the same at on the bottom half.
Just take a scalpel, razor blade, whatever... and cut the stitching at the bottom where it is stitched together making the loop that is holding the swivel.
Take the swivel off, then loop the sling through the slot in the stock, and back through the other plastic piece that tightens it.
Then when you want to put it back on the regular stock you just undo it, loop it back through the swivel, and tighten it back up again with the plastic piece (wish I could remember what that thing is called)
PS.. I had him put that swivel on the stud of the regular stock on his SBE so he doesn't lose it.