You can on Poloniex and Kraken.....I can't get onto Poloniex so can't speak on it, but using Kraken is an exercise in futility and a test of patience that would cause even Jesus himself would throw stones through the monitor.

BTSdex and Openledger.io allow you to trade to BitUSD, which is actually backed 1:1 with USD, unlike USDT which fluctuates in price. It has a small learning curve, but I'm sworn off all other exchanges at this point. I guess I'll just have to miss out on coins that openledger doesn't support.....and I'm ok with that.....that's how bad Kraken is. Server error 8/10 times you click a link or submit an order.