I have seen it done with prepo's Helsinki Central Station and FF's Kings Cross Station for example. I tried transit-enabling using the TE editor (SC4Tool) as well as adding the Occupant Group for subway in the Exemplar editor (SC4Tool) but to no avail. I've looked extensively for help but have found none. Please help me if you can. Thanks!
It isn't really that difficult. :)
If the building already is a station of some sort, you only need to add one rep (value) to the OccupantGroups property: 0x1302
That will identify it also as a subway station (Building: Subway).
You also need to check the Transit Switch Point property.
When doing that, mind that the subway station is located "inside" the lot
(any subway track that is built under the lot is considered inside the transit switch).
Thus, you may allow pedestrians and train passengers to switch directly to subway from the outside to the inside and v/v.