That's impossible to do, as CDMA and GSM are two completely different technologies. There's no way a factory unlocked (GSM) phone can support CDMA as it's not physically built for the network.
Yes it is. The iPhone 4S has the same insides for both Verizon/Sprint as it does for any GSM/UMTS carrier. Both Verizon and Sprint iPhone 4S can be unlocked to use a micro SIM. The issue though, is they won't unlock it for use on domestic networks, only international.
As to the original question, You can use Cricket and I believe they'll flash your iPhone for you. Verizon and Sprint both will only allow iPhones sold by them on their network. Although I have an unlocked iPhone 4S, it will never work on either Verizon or Sprint, not because it's technologically impossible, but because both carriers are restricting the devices allowed on their networks.
If the CDMA phone is a world phone (it has a sim card), the phone's GSM can be unlocked by the carrier and used on GSM networks. Sprints phones, except the iPhone is unlocked by default. You have to call sprint to unlock the iPhone and they will only do it if account is in good standing.
My 4s is Verizon and only unlocked for international gsm .. Like stated yes by theory the phone it self is capable to use gsm carriers abs CDMA carriers but the carriers but restrictions so this doesn't happen
CDMA carriers block esn not sold by them and will unlock the gsm slot only for international sims
Gsm carriers can unlock the phone for any gsm carrier but because the phone was not sold by a CDMA carrier that phones esn can not be used on a CDMA carrier