That is the opposite of what you asked in your first post, so now I don't know what you want to do.
If you obtained a free inbound Google Voice number from Google, and now you want to port your mobile number in to your Google Voice account, and use that number as your inbound Google Voice number, yes, you can do that. If you want to port OUT a free inbound Google Voice number to some other carrier, yes, you can do that.
If you clarify your situation, perhaps I can provide a more meaningful answer.