Thanks! This is practically what I was looking for the whole time.If you want to copy an entire gameobject, rather a specific attribute, you can duplicate it with "instantiate" button, at the top of the inspector also next to a copy button. Note that the duplicate will not be parented to anything, and will appear at the bottom of the gameobject explorer by itself. It will still mostly work like the original.
Though if what you're trying to do is simply changing the model of a shirt, you can do that by clicking the thing that says something like "SkinnedMeshRenderer" in the shirt you want, copying the "sharedmesh" attribute, and then pasting that onto the shirt that onyx currently has.
Basically what I was trying to do is copying a model and then parenting it to another model. E.g. I remove a shirt from an Onyx rig and then want copy a shirt from another rig and move it to the one with the missing shirt. Now with the instantiate button I can duplicate items and now I can assign them parents (altough I have to type them in manually).
Practically now I can just remove stuff and readd them if I needed with the freecam mod, so thanks