I assume by "category" you mean a group of three performances (like "oral", "dancing", stuff like that). I'd have to think about it but I believe it wouldn't be easy to add. Game mechanics wise it would change the pace and logic and the game. Technically, the game was made for 6 categories so adding one would be a rework of a lot of code. As for the girlpacks, this would be a lot of work for the packmakers (as for what I understand, there are more than 500 packs at this point...).
Adding just another performance (like cowgirl in the "sex" category) would be a lot of technical work too, plus an imbalance between categories(as it would mean that one of them would have 4 performances and the other only 3).
The exception here is for finishers. Adding one shouldn't be a big issue (in fact, I already partially added one in the next update) and wouldn't change the game mechanics much. The only problem left is for packmakers, as it would be a lot of work for them (again, more than 500 packs). Plus, I don't know if I should make this finisher available by default (meaning that for the moment, all the packs would use the default clips for it) or not.
All in all, modifying the packs in any way is a bit of a pain and a lot of work for me and the packmakers. Just changing names though (like renaming the "Missionary" performance to something that would include different positions) is an easier solution as it would give the packmakers the possibility to redo their packs if they want without being an issue if they don't. Plus some packs already include different positions in the "Missionary" performance.
That is what I'm thinking right now, and that is why I'm hesitant to do it. When I said adding a Cowgirl performance would be doable, I didn't give it enough thought and didn't realize how complex (both technically and for the game mechanics) it would be.
tl;dr Technically, the game is made for 6 categories and 3 performances per category, there are a ton of girlpacks, and changing that would require a lot of work, from me and packmakers.