You can ask both the TA and the AC AAngels to put you on the volunteer list. However, the GA is the only one who knows if you will be needed or not.

I have had TA and AAngels refuse to put me on the list because they didn't think the seat would be needed. However, when they do put me on the list, it's convenient because I don't need to rush to the gate to be first on the list in case they only need one or two seats.