I found out how to create clothing items. But I am not actually "modding", but fiddling with some files of the base game on my own, actually changing/adding files and keep bkps for the entries.
You need to know where the files are located.
media\fileGuidTable.xml
contains the entries of unique ID numbers.
media\clothing\clothingItems
containts the xml files, refers to used models, the unique ID etc.,
media\textures
contains the image files
media\scripts\clothing
contains the .txt files for the game how to handle/use it, like DisplayName, BodyLocation, used Icons and several attributes.
With these steps you have an item in game, which you can spawn with a cheat mod/item spawner.
There are several files, where you need to add lines, so your item can spawn on zeds, in containers etc.
It has been a while, so I am not sure where exactly.