Yes, it is. You can use all convertible currencies -as well as international credit cards-in the Souks. It is good to know that the exchange rates in Morocco are regulated by the state central bank. Therefore, there is no black market and the rate for cash exchange is the same wherever you do it: airports, banks, hotels, shops, etc... There's a 10 Dh fee for every traveler's check to be exchanged,however. Needles to say that it recommended to have the local money(Dirham) to cover small expenses.