Can I add "Buy It Now" to an existing auction?

[blueridgebear]

I've had an offer (via an eBay message) on an item I'm selling and it's high enough to be interesting. I have no current bids on the item. There are more than 12 hours left on the auction. Ebay's help thing is useless--it goes round and round in circles, never answering the question. It says you can't change the format, but that leaves hanging the question of whether adding BIN changes the "format." I googled and got contradictory answers, which doesn't help either. Can I get a simple, accurate, unequivocal answer, please? This is getting frustrating and I've got a sick kid, so I can't justify spending three hours trying to answer what I thought would be a simple question.