I'm a new creator and I wanted to ask the professionals if they have any ideas on how I can pull this off.

I really want to be able to add the hybrid form skin from the movie Underworld to Sims who are infected with both lykathropy and vampirism. The mod would make it so the Sim transforms from any default werewolf skin into a custom one, BUT this transformation only occurs when the sim also has vampirism.

I have had this idea for a while but haven't any clue as to what to edit or where to begin, any direction you can provide or if this is even possible would be greatly appreciated.

So far my guess is that I would have to add a custom mod that overwrites the werewolf skin in the event it detects vampirism as well.

Well, for starters, I'd say you'd want to look into the custom body overlay technolergy from Morague. Once you've created such an overlay, then it would be a simple matter of adding switch case to the werewolf transform to use that one instead.

Thanks, its a good area to start