Give user choice of domain

Is there someway to give the user a choice of domains when they sign up?

The idea, is that they would choose where they want their sub-domain to be. For example, if they want the sub-domain 'joe', then they would have a pull-down menu, giving them a list of the possible domains they could choose.

It would be good if this could be done, so you could have multiple users called Joe, depending on the sub-domain selected.

If this isn't possible, is it possible to have the user name automatically go to all available domains, so when Joe signs up, he will have that sub-domain on all the domains available?

Thanks