I'll address it, if you are being serious.
Former HOF coaches, the janitor, or even you and I can attend practices as long as the head coach is okay with it.* As far as I am aware there is no prohibition against it. There are only a limited number of people who can coach players, but if Calhoun wanted to sit every practice and then kibbutz with Hurley afterwards, there isn't any NCAA rule that I am aware of that would prevent it.
*If the head coach is not okay with it, obviously, you will not be staying. I speak from experience here, although I have to say Hobbsy was very nice (almost apologetic) about it.