Tattoos and custom skins/skin details layering

Hi. I don't know if this is the right topic for my problem but here it is: I created some moles for a certain sim in my game and when I try to use them with some custom skins that I downloaded, they just replace the respective skins. Is there something I can do so that I can use my moles with the custom skins that I downloaded? Please, can someone help me?