Are exchange rates available to use in a workflow?

The business would like to have a calculation to determine the difficulty of a sales opportunity.  We are a global company and are using multi-currency, for the calculation we are using multiple fields and one is the Annual Sales Potential which is entered based on the currency of the client.  For the workflow calculation we would like to convert this amount.to USD.  We enter currencies for all currencies used in the business and from my research this appears to only be used in the reporting and not available for calculations. 

Any help would be greatly appreciated.  TIA