I'm kind of new to wifi stuff and I've never used a wifi extender. Before I buy one I would like to know if what I want to do is even possible.

What I want is: I want to use an extender to boost the signal from an xfinity public access AP. I have an xfinity account so I can log in to the AP if I am close enough to it (standing on the edge of the balcony outside my office). I don't know where this AP is located and I don't have any info such as MAC address or other ID numbers for the AP.
Does the AP broadcast those numbers?
Will it be possible for me to use a wifi extender to boost this signal into my office without knowing anything about the access point?