In the US, Verizon will only activate phones on its network that it originally sold. I think they have some other program where they will activate other phones of certain models, but I'm not aware of anyone successfully getting them to do so.

Sprint may have similar policies. I don't know.

Nonetheless, the Verizon iPhone 5s will do CDMA roaming on CDMA networks in the US and overseas per Verizon's roaming agreements.