Clothing categories and separates
Sadly the answer is no, it's not possible. At least not that anyone has figured out yet. Personally I've tried making an "everyday" separate and changing its category to PJs or something in SimPE, but it didn't work.
So @Oeselian, I'd say give it a try, absolutely! And yes, please do let us know how it goes.
Really wish this info had been around back when I was making oodles of clothes. Hmmm.... and now I have an idea for something that I want to try....
* fanseelamb scurries off to attempt a thing
ETA: Also, a little tip - if you need to add up hex numbers like it explains in that second link I gave, you can easily do so with the Windows calculator. Under the "View" menu, choose "Programmer" (instead of Standard) and then click the "Hex" radio button on the left of the calculator.