@Creepypasta Art you can add hat through unity, you have to export the vrm file, after on unity you have to import the univrm package for let unity import vrm file, then you need only two files:
1 - the vrm saved before
2 - the accessories (hat/piercing/ecc) in blend file or obj/fbx
For final step load the vrm and accessories on unity, then scale and move the accessories on the part on the chara you like to have it, for this example the hat go to the head (sorry for saying this even if it's obvious, but it's important for the next one), when move and scaled perfectly, link (parent) the accessory to the bone called head (by drag and drop). For make sure it's doesn't move, you can select the chara from the tree view of unity and move it trow the 3d arrows (the lower one). Now you can export it always by the univrm. Remember only a thing when export it to vrm you can't import again to vroid. Sorry if this "tutorial" seems a bit messed up, but after a while, it's pretty easy.