ok, where there's a will there's a way..
I don't have any personal experience with this, but you can try to take over someone's plan, maybe someone here upgrading to a pre? I know i've seen sero plans offered up for sale. I don't think you have to have a fam plan, as long as you qualify for 2 lines. I think the trick is the other person has to qualify too, otherwise they would have to do a simple change of plans. or of course if they were just going to swich carriers.
you can do a # change for free.
another thought would be to give liberty wireless a call, maybe you'll get lucky and they'll accept the esn. (they resell sprint service on prepaid)