If you are not going to use the Females strip to rude option or you all-ready have the bodys set up.
Note: To make your own, I recomend useing bodyslide to make your dream girl, export your body...
Rename
(Skyrim)Datameshesactorscharactercharacter assetsfemalebody_0.nif
(Skyrim)Datameshesactorscharactercharacter assetsfemalebody_1.nif
To
(Skyrim)Datameshesactorscharactercharacter assetsfemalebody_rude_0.nif
(Skyrim)Datameshesactorscharactercharacter assetsfemalebody_rude_1.nif
Then you change to a body with undies and export.
You will end up with woman having undies on normaly and a naked body for use in sex.