yea 5dimes allows it. basically when u want to put in the other part of the wager you click "straight and RIF", you then pick the play you want and when you get to the screen where you would normally enter the amount there is a red "IF" circle to the right. if you click that it will show you which bets are able to be converted from straight wagers to if bets. it is very handy in case a game isnt going to end in time but you have it locked up and want to use the proceeds for something else
