The error message that was displayed to you, "This number appears to be from an area we don't currently support for porting" means what it says, literally.

Number porting background:  Google relies on a third-party telephone company for its telephone network services (providing new phone numbers, hosting ported-in numbers, and making and receiving calls on the public switched telephone network (PSTN)).  After the "Bell System" was deregulated years ago, many Competitive Local Exchange Carriers, or CLECs, started offering service in various parts of the country, in competition with the incumbent carriers or ILECs. They do not offer, nor are they required by the FCC to offer, service in every single local telephone exchange, or "rate center".  Google's CLEC partner simply doesn't have a presence in the rate center for the number you want to port in to GV (they don't have a port on a telephone switch in that exchange). How do you know that? That is what that specific error message means, as a result of performing a search on your number.

Numbers are grouped into blocks, identified by NPA-NXX-NXXX, where NPA=area code, NXX=prefix, and the N in NXXX is the block number.  Portability is determined at the block level, which is within a local telephone calling exchange, or rate center. Since the block number is a subset of the entire, ten-digit phone number, this means that a "thousands block" includes the nearly 1000 numbers defined by the first seven digits of the ten digit number, e.g. 415-456-7XXX is a block, and 415-456-8XXX is another block.  Each local prefix can have up to ten blocks, zero through nine. Certain rate center prefixes are controlled by a single LEC, in one block of nearly 10,000 numbers. An area code typically has many prefixes. Thus, it means nothing to compare some friend's or relative's number in the same area code, to your number, with regard to portability.

There is nothing you can do about it.  Google can't tell you if it will ever be able to port in your particular number, because it depends on the business operations of an independent, third-party company (the CLEC), and that company doesn't publish future plans for expansion.  The third party also leases their services to other customers, so it would be useless for Google to publish any sort of list, that they don't exclusively control. Porting your number first into some other carrier does nothing. The portability of the number is based upon access to the telephone switch in the telephone exchange that would need to host it, and which CLECs do business there, not just the carrier that holds the number today.  The fact that some other carrier(s) can port in the number has absolutely no bearing on whether Google's carrier can do it.

Telephone numbers are the property of the original, issuing Local Exchange Carrier (ILEC or CLEC), who was issued that number block by the North American Numbering Plan Administration, or NANPA.  Carriers lend their numbers to other carriers via number portability, but the number itself is still controlled by the original carrier. That is why it makes no difference if it is ported elsewhere first.

Everything you ever wanted to know about number porting is here, from the source:  the organization that is responsible for all number porting in the US, the Number Portability Administration Center or NPAC:

Google and its CLEC partner must follow FCC regulations and procedures, as administered by the NANPA and the NPAC.