You can intro a lone glider to a pair. I intro'ed Jasper & Zoey who have been together since they were 8wks.... to my Kodah who had been alone since he was 8wks....I got him when he was 5mo.
Just do it one on one...don't keep the bonded pair together and try tossing a lone glider in with them or even do an intro with them together...they could gang up on the single glider.
I did Jasper & Kodah...let them sleep together in the pouch all day ON ME...so I could monitor. They went back to their prospective cages that night.
Next morning I did Zoey & Kodah, again letting them sleep on me for the day. Back to their own cages that night, then the next morning I took them all 3 into the tent. They dove into the pouch and slept together all day. I cleaned the cage they would share and put them into it for the night. I stayed up all night making sure no fighting broke out. They've been together since.