Yes, it is possible, but not out of the box. You'd need some sort of custom plugin for this to work, and it would hook out to some sort of payment processor to handle everything. Did you have something in mind?
They should purchase more SIM cards to create more accounts
In some countries its actually really easy to get ahold of a SIM card. I'm from Malaysia and when I go there I can get a throwaway SIM card from any convenience store in about 10 minutes for a couple bucks
EDIT: Meanwhile in Canada.... :rage1:
They're free in the UK, but need an initial top up to activate, £1 would do this on some networks.