Physically impossible. Between the I-93 decks' support pegs and the Orange Line viaduct not touching down at ground level until it's underneath the Cambridge St. overpass there's no way to worm a commuter rail track over to the 3rd OL platform at proper track geometry and no way to widen anywhere else for a new platform. It's a thoroughly fixed space. There is room on the other side of Maffa Way, but that would've only worked as a superstation if the pre-'75 Sullivan station property were retrofitted for the new alignment. It's a functionally useless block away from the OL platforms, busway, and entrance.
This is why the first CR station on the cleanroomed 1975 ROW stayed at Malden and not Sullivan in the first place. 93 came first and everything...the pit, the viaduct...was shaped by those deck pegs. This Sullivan CR idea didn't escape them; they considered it, revisited the idea years later, and rearranging things between the pegs never washed in any way engineerable in non-broken fashion. Just because you can squint on Street View from the overpasses and conclude "This might work if you nudge X, Y, and Z" doesn't mean it'll work at track geometry that'll preserve service levels on a mainline lead that has to serve 2 masters. If the turnouts can only handle a 5 MPH platform approach you triple-kill every achievable headway to Reading, Newburyport, and Rockport at the same time. That's not an acceptable tradeoff for cramming 3 CR stations for 5 OL stations, or 2 for 3 out of North Station.
