I think it's perfectly okay to ask. If your T doesn't have availability, then T can talk to you on the phone.

I just recently asked T for a few extra sessions, she was almost excited! I was joking with a friend that she must want the extra business, but my friend said that it's a big deal for a patient to ask for an extra session, and that it's also a big deal for your T to arrange that for you because your T wants to be there for you.