Using CCDEX on DEX you are able to exchange USD for bitUSD, which is an asset on the BitShares platform pegged to the dollar. This can then be readily exchanged into any other asset on the platform. Other than USD, you can also do this with Euro and, from memory, Chinese Yuan

To use CCDEK, you will need to prove your identity before you can use it. Once you are verified, deposits are free and withdrawals are charged at a 3% fee.