Not possible due to hardware differences. A LINE port does not provide the voltages necessary to run a phone, but will accept and react to voltage from another source. A PHONE port provides voltage to the phones, but usually can't tolerate any voltage from another source (such as a PSTN line).
To use a bad analogy, it's like saying, "I'd like to be able to use my garage as a car!" Although they have related functions, they are totally different from a physical standpoint.
What I'd like to see someday is Obihai come out with a device that has four (or even eight) phone ports, so you could run four-line PSTN phones or (if used with an Asterisk or FreeSWITCH server) have a separate extension in every room, etc. If they're going to make a multi-PHONE-port device, I'd like to have more than just two.