try this: https://www.gta5-mods.com/tools/meta-toolkit
To my knowledge, from what I read in the comments of a post you can create more clothing slots with this tool. "Copy and paste more lines in the file "mp_m_freemode_01.ymt" at around line 6464 to add more tops (jbib) to the game."
Also this might be of use: https://forums.gta5-mods.com/topic/12914/openiv-add-on-modloader-tutorial/2
If that don't work, check these two links:
The Emergency Uniform Pack is allegedly an addon mod (although I haven't downloaded it to see) which uses multiplayer clothing, they're streaming the addon mods through NativeUI to use on FiveM.
They also provide outfit hash's if you need them; https://docs.google.com/document/d/1lJNZGGN9h0EXNE8evtDZvVJhkN7jPVPmsU-hNiKnpL8/edit
This video might also help in relation to EUP. Not sure, just trying to give you some frame of reference for a better understanding: https://www.youtube.com/watch?v=qxjgP--fM_w