Ha ha, Cheat--- Just find a clip of some audience from another recording.
Applause often blows my levels over the top when doing acoustic type recordings so I have the opposite problem and have to edit the levels way down.
A condenser mike(s) will work and with experimenting they can even serve double duty as room mikes for the band too. The trick is to get them away from the speaker stacks somehow so it doesn't blow the levels to high.
Best results for recordings at live gigs I get with my Tascam DR-40.
Place it FOH on a camera tripod. Mine is 4 track so the FOH mix goes in XLR and the ambient mix via the on board mikes. If you were way back in the room you just have to time shift the mike tracks until you get a nice room delay.