There are few tricks that I've tried but can't really confirm on success such as ;
The flash light trick ( Aim a light directly on the BTA at night so the clown can see that it's like a beacon )
The Picture trick ( Putting up pictures of clowns hosting in anemones around your tank, so a sort of Monkey- see- Monkey- Do thing )
Or the food trick ( Placing food strategically around the anemone so the clown can go by it and maybe spark some interest in it)
But ultimately it depends on the clown and anemone they choose if they are right for each other ( so to speak) and it's really better if you don't try to rush things, Either or might end up losing their life. No they aren't natural anemone/ clown pairings, Yes it can happen that they pair up ( Currently happening for me)
Well hope this helps and Good Luck! : )