Why, with another of the fine tools that came out of the GWX project, the GWX Terrain Extractor:
Select the proper grid square, and then open the exported RAW image data in something like PhotoShop, change it to grey scale, then indexed color, chose the proper color set in the game folder, and then edit to your heart's content. Just stay 6 squares away from the edges to avoid having to edit the "neighbor" grids. More thorough instructions included in the package:
https://www.subsim.com/radioroom/dow...o=file&id=3526
One important thing to remember is that Alcatraz is not that "tall" in height above sea level as opposed to its surface area, so you most likely will not get that accurate of an "island" showing, and then to find an appropriate enough building structure... My guess as to why it is not there is a combination of that, as well as its being basically in the "traffic" lane in and out of the harbor...
Another way you could do it is to do something like Lot's Wife, and put that on top of some 'raised' terrain as above, which might get you a better looking "Alcatraz" than just terrain alone... dependent upon your (or someone else's) "art" skills of course... lol