Normally you would hide the genitals and use a magent of you want some kind of bulge.  Since it is hidden that would not make much difference/

However - it should be possible in the clothroom.
What you could do is to  have the genitals at frame 1 at a scale of 0 and then increase that scale to 100 on frame 10 - all while the figure remains in zero pose. Then set your final pose at a later frame and calculate the whole thing.

Whether it looks good or not, depends on the amount of vertices in the cloth object in that area. There might nog be enough of them too stretch that far