Hi Ron:
Google Voice accounts can have a maximum of two inbound GV numbers, as long as one or both of them are ported in, vs. being issued by Google. So, yes, you could port in your number, but GV can't accept ports from land line carriers. You'd need to first port the land line number to a prepaid mobile phone carrier, and wait a few days, then port it to GV. The result would be that calls to either number would ring the same destinations you have selected on the Phones tab of GV Settings,
https://www.google.com/voice#phonesCalls to either number are handled the same way (there is no way to have different greetings or different phones ringing). If you want to treat this as a separate line, then instead of porting the number into your existing account, port it into a new/different Google account. You would then set up that new account as a separate service provider slot on your OBi.
For further advice, you can come on over to the Google Voice help forum, linked in my signature below.