It's technically possible to "unlock" a CDMA phone, but in the USA, CDMA carriers refuse to unlock a phone on their network. And even if you did get an unlocked CDMA phone, the providers will refuse to activate any phones that were not originally activated on their network.
I use Verizon and they are always more than happy to activate my CDMA capable phone for use on the Verizon network - which I have been using for a very long time and live contract free this way. I also use a very old completely Unlimited plan, so buying a phone from Verizon would cancel this plan and force me on a new contract.
Anyhow my point is I have no problems adding a personal device to my plan anytime - many times.