If you want to add things like piercings to the breasts, the process is simple. All you need to do is name the prims correctly and link them to the breasts in the desired spot, and you are done.
As far as the naming scheme, each prim has to be named "breasts, <prim type". So for example a piercing would be called "breasts, piercing" whilst a piece belonging to the bra would be called "breasts, bra". Furter supported types are "clothing", "cumcover", "nipple", and "breast". These pieces will then show/hide according to the settings selected in the HUD.