Verizon really does not let you add another device to their network unless it was originally made for Verizon....
To understand unlocked phones and what not,
A locked phone means that the device will only work on a specific carriers network.
Once a phone is unlocked it will work on any carriers network provided that they are using the same protocols.
The 2 protocols used in the US:
GSM = ATT, T-mobile, others
CDMA = Verizon, Sprint, Alltell, MetroPCS, Cricket, others
Most parts of the world are GSM. Those phones you see online that come from other parts of the world are most likely GSM phones.
The problem with buying unlocked GSM phones is that other parts of the world use different frequencies for voice and data.
Read this article
http://en.wikipedia.org/wiki/GSM_frequency_ranges