Tyre modding is not currently supported, but it is possible to create new tyre treads and sidewalls via a Custom Paint.
If you would like to do so, create an instance of
MI_Rubber_Tyre, located in
Cars/Wheels/TyreMaterials, and replace the tread and sidewall textures with your own. Note that some of the tread and sidewall textures are packed (they contain multiple different masks in their RGB channels.)
The sidewall and tread each require two textures: a normal map, and a packed "OHT" map, which is laid out like so: Red for ambient occlusion, Green for the heightmap, and Blue for tessellation density.
The sidewall also contains another packed map for the whitewall and lettering masks. This map is currently unused in-game, but the settings are available in the MI_Rubber_Tyre material if you wish to enable them for your Custom Paint material. This map is packed like so: Red for the lettering mask and Blue for the whitewall mask (with Green being unused).