Unity cloth is an upcoming feature that will be support by UMA.
Creating a cloth weight slot
To create a slot data asset that has cloth weights on it.
1) First, from your FBX file, create a scene object from it.
2) Add a cloth component to your mesh.
3) Set the vertices weights for the cloth.
4) Create the slot with the slot builder. Currently, the "Slot Mesh" field will not accept your scene placed mesh, but you can drag and drop it on the "Drag Meshes here" area. You will need to set it's material to a cloth enabled material. See section "Creating an UMA Material with cloth".
Creating an UMA Material with cloth
1) Create a new UMA Material. Right click, Create->UMA->Core->Material
2) Select your unity material. Usually, this is "UMA_Mat_Diffuse_Normal_Metallic" and set your channels. For advanced users, this is where a double sided material could be set up, important for cloth objects.
3) Create an UMA Cloth Properties. Right click, Create->UMA->Misc->Cloth Properties. Set your default cloth settings.
5) Make sure any slots (that have cloth weights added) and overlays' material are set to this new UMA Material
Your slot should now automatically have a cloth component added with it's weights set when active in a scene.