If you want to add a new currency to your OpenCart : Follow this post
My default currency is INR, Lets see how we can change this to USD.
1. Login to Admin Console.
2. Goto System -> Settings
3. Edit the Store Name for which you want to change the default currency.
4. Now, Select the Currency that you want as Default.
5. Now if you see the User Page , the default currency will be updated.