I think they quite possibly could be, the light switch can be purchased individually as can the light sensor to trigger them but not sure about fitting as the standard sensor lives in the rear view mirror so that may need to be changed but could go in loose for testing
From memory the replacement light switch and sensor are under a hundred pounds but as with everything this new if we cant find anyone else thats done it then its always a bit of a gamble, i took a chance on my rear view camera and retro fitting rear LED lights but they both ended up going in fine and fully working
Where abouts in the world are you ?