The only way to apply materials to a certain spot would be to have the asset split up into multiple objects for example you have a room and you separate it into walls, roof, and floor. A different way to achieve the same thing would be to take the asset you have right now and export it into Blender then you can assign materials on the parts of the asset you want, bring it back into Unreal, and apply the materials you want to the already assigned material from Blender. Hope this helps :>