To add your default base currency, go to Settings>Persoanal Settings > Expenses. . You will see a drop down area to select your Base Currency.
You can also add a Currency Rate Increase. Example is if you get charged 4% from your credit card provider for all foreign transactions. Simply add the percentage and any foreign currency added will include this rate increase. Rate increase on calculated when an expense is added to an expense report.
In the above example, any foreign expense will be subject to a 10% rate increase when added to an expense report.