Is it possible to ask offers of admission to extend the deadline?
Yes, it's definitely okay to ask.  They're certainly not going to rescind the offer because you ask for more time.  Depending on the school, their applicant pool, how bad they want you, and (probably) how rigid their administrative structure is, they may grant or refuse your request.

You should, of course, word your request very politely and recognize that they have the right to say "no".