You cant add items that the programmer has not put in. However you CAN paste on texture files on top of the textures used. I used that affect with a tounge stud on one character, and it came out quite well.
Not sure how well it would work with labia rings, but its worth trying at least. The only difficulty is getting the skin of the texture map to match with the surrounding texture. Ive never found that particularly easy, but you can always try with a new raster.