Tyre modding is not currently supported, but it is possible to create new tyre treads and sidewalls, by creating them as 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 the tread and sidewall textures are packed (they contain multiple different masks in their R,G,B channels.)
The sidewall and tread each require two textures: a Normal Map, and a packed map, which are laid out like so: R - Ambient Occlusion, G - Heightmap, B - Tessellation Density
The sidewall also contains another packed map, for the whitewall and lettering masks. This map is currently un-used 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: R - Lettering Mask, G - [unused], B - Whitewall mask.