That's gonna be hard as crap. You're gonna be working with some severely thin metal to adapt that since you can't make the barrel any longer and get the suppressor to fit.
Well that does give me an idea.
If you found a 5/8-24 to 3/4-24 thread adapter with the correct outside diameter, I guess you could cut the barrel down to the 19.x or whatever it takes to make barrel + adapter equal a full 20". That could work but it would be ugly.
Otherwise, check Charlie's Custom Clones or the EE and see if an M110 barrel turns up. Happens every once and a while, especially since Charlie got a run of them made.
ETA--ADCO could probably do the cut-down adapter thing, and it anyone could do your thread insert they'd know. I'd call them and explain your problem.