As others have stated I've had luck with getting companions added at the gate, but certainly not before given the written policy is pretty clear as mgcsinc pointed out.

Often times the GA will even let you know if it's worth it or not. I've had GA's tell me... "I can add them but there is only one seat left, and there is a 1k on a higher fare class that has already checked in... I'll add it if you want but it's highly unlikely they'll clear."