My case : I wanted to add some more EUP outfits to the LSPDFR Police locker so that first these outfits can be used for random officers and LSPDFR backup (not Ultimate Backup). So i added new <Outfit>...</Outfit> lines in the outfits_lspd_eup.xml file in the Custom folder, but it didn't worked.
Solution : After some long research, I found out that you need to add the script name that you gave to your outfit, in the <Peds>...</Peds> section (like the other ones listed where it says "for outfits selection" (careful to the gender character)) in the correct agency inside the agency_eup.xml, again in your Custom folder.