I do have the non-default version..it's a nude top a would like to make it into a preset but I can't see any tutorials
Well... That seems tough one ... To my understanding converting something means you change it into another category for cas like default eyes changing into make up or face paint for example. The nude top you mention the only way i can think of is changing it into tattoos (so they will not override possible clothing) and the following problems occuring:
1) Need to use a program like blend and separate the top into all different areas of tattoos that the studio supports accurately and that means you need to retexture edges also along with finding the correct areas and sizes.
2) You will need to do a lot of them in recolouring for every skins and skintones you are using in your game so they will not showing out of place since they will be fixed and not optimized from skin options any longer.
3) Personally on nude parts haven't yet found ids to change since all of them appear as straight zeros (unique id is needed so the cc is addressing the correct game address and not conflicting with other things).
Not sure if more experienced people have some breakthrough in this but for my limited knowledge this project does not appear a shane idea for so much effort and so little to give.
PS TURBOBIT from ww mod created something similiar for male genitals part to work as tattoos although this area had less space to cover and was not multiparted like an entire top. try contact him since he helps a lot of modders with their problems but seriously consider doing it only for breasts for example XD.