I'll see if I can find the document with that plan.
Looking at it from a civil engineering perspective, I think that connecting deep tunnels to the existing station tracks would be an engineer's nightmare. Given that the new station is going to be beneath the level of the existing station, there would have to be a switch way out under the river and a parallel tunnel just to go up that addition 20-30 feet or more. You're talking about at least a third of a mile for that change in elevation, which means six tracks under the river at the west shoreline of Manhattan.
