It is not hard to add another set. I did it just now and I think I needed to modify 5 different sections of code to add in a "adult" category. But that is the easy part. You would then need to add and modify another thousand pictures before the game works nicely and when I checked most of my models are not pornstars and many don't even do solo.
What I was wanting to do is have "custom" categories so that you could call the sets whatever you wanted like "cosplay" or "swimsuit" but that would require significant modification.
The sections to change off the top of my head I can think of are:
1. Adding button (you did this)
2. Adding array to $Shoot - to specify the "050" filename, income and income modifier for the category
3. The different categories have different chances of getting different types of cards so you would need to add the additional info for $ShootType
4. Adding the new category to the girls $Bio which is dynamically created at the start of the game so you have to add it to _TempModelInfo. This is so the game can record what cards you have gotten already for each girl in each category.
5. Adding the display of the cards in the portfolio screen (just copy and paste one of the other lines)
I thought I counted 7 sections to modify, hmm I hope I haven't messed up my code changing it back. Most of the above changes are just 1 line of code.
6. Adding this category into the achievement check (not necessary)
The errors you are getting on the previous screen are because you have not done number 2.