Seems GSM carriers tell me that the number is 'tied' to the GSM SIM, so not possible. Anyone know if this is the case for CDMA too? Q: Can I port in a number to a line a few months after signing up?
Yes, you can port in a number to Sprint (or to most Sprint MVNOs) over the top of a pre-existing number, without impacting the Sprint line or plan-- including the one year free plan. (And the number ported in later, over the top of a pre-existing number, can come from a Sprint MVNO without issue.)
I think some GSM carriers don't object to this, either, although you have to buy a new SIM.