there is, but not a good one yet... you need to exchange manna to btc on for example south exchange, then transfer it to for example poloniex, then exchange there btc to steem dollar.... then transfer it to steem...
If you want to lower transaction fees, then take these step in between: exchange btc to for examle dash, or bitcoin casch transfer dash / bitcoin casch to poloniex, then exchange back to btc...
Or much easier find someone who is ready to exchange directly :)
RE: Brandon Venetta invites all Steemians to a Manna Meet-Up