I imagine that it wouldn't be impossible, but it would certainly be needlessly difficult to add a station in an underground segment. The road network in the Westhill area is entirely unsuitable for a proper TOD.
The designated place for Port Moody's third station is at Clarke & Queens, between the switches for the tunnel's lower portal and Moody Centre station:
There's only about six blocks between the location at Queens Street and Moody Centre, so I can't imagine Translink wanting to go ahead with it without considerable density being added in the immediate area. It might make a little more sense in conjunction with the Westport Village project on the former Andres Wines site to the west, but it has been very slow going, much like everything in Port Nimby.
A key difference between this station and the one at Lincoln, however, is that the line is already built, when it had not yet been when Lincoln was negotiated.