Addon clothing doesn't work

I am trying to make an addon clothing pack with this resource for my FiveM server:

I don’t want the uniforms to replace the original GTA 5 clothing, but simply add it on top of it.

I’ve tried it several times with GTAUtil, but the clothes still partly replace the original clothes or don’t load at all. Does anyone know how I can get it right, or does someone have this pack working as an addon?