No. You can buy an unlocked phone that is the same model and use it for another network but typically a Sprint phone will only work with Sprint and vise versa.
Unfortunately you can't. Verizon uses and EVDO signal whereas Sprint, AT&T, T-Mobile, etc... all use CDMA signal. If you want an AT&T phone on Sprint you would have to buy a SIM card. But...
Depending on the make and model of the Verizon phone you would be able to use it on nTelos services. The best bet would to be to find this information and then call nTelos Customer Care at...