This has been answered multiple times already.
The 3D model for the boys doesn't include any "junk", and we have NO way of editing the models themselves. The best you could do is "paint" a dong on the skin texture, which would look beyond stupid. Its dirt-easy to do females, because the breasts are usually already there and you can "fake" a nipple with a normal map, and downstairs girls are "innies" rather than "outies", so creative painting can create the illusion of labia.
Putting a penis where there isn't already one would be like giving someone a 3rd arm. There's just no geometry to work with.