Whether you need to convert to just one other type of currency, or many, you can start with a Calculated Column in your list. For example, to convert from one type to another type, when you create your list:
1 - create one column of NUMBER type. This is where the user enters in a number (for whatever currency you're converting FROM). Then...
2 - create another column of CALCULATED type. In the FORMULA field, enter in your equation based on your first column. For example, you'd put something like:
=[USD] / 2
..or whatever the conversion equation is for the currency you're converting to.
You can choose it to come out as either currency or number, whichever works best for you.