Firstly, it would require to make it in a way so lobby is still balanced, adding semi-random 2 ships is almost certain to break the balance (given how many people crewform).
Secondly once I asked Muse about option for players in custom lobby to transform lobby from 2v2 into 3v3 with a button and maybe some voting, and they basically said it would be hard and they're not going to do it.