The Navi definitely has this option since I have done it with mine, the drivers door now unlocks all doors on mine. I know that Mitsubishi dealerships can program it if you do not have the Navi.

 

i got mine changed at my last oil change, it was free. way better this way.

 

I take it you don't have this feature?

The thing is , once you are used to the FAST Key feature i.e not having the remote in your hand but buried deep somewhere in the pocket, its a nuisance to start fetching the remote just to unlock the passenger door only. Thats the whole purpose of that feature, not to use the remote/key to get in the car and drive. Till to date, i used my remote only 5-10 times, other times, I use the door handle button to lock and FASTkey to unlock.
Not to forget the remote has lousy weak coverage to get a head start to open the door before you get to the car. You have to be literally less than 5metres away from it so that the car receives the signal.