Why not create a table that holds the names of the currencies that you will converting to and include in that table the currency conversion rate?
Whenever you decide to run the report, you should be able to add a variable into the fields themselves that will represent the multiplier and then retrieve that multiplier by a form or with a combobox choice.
This is just kind of the theory of it, since that was drift that I caught of what you were asking. The specifics would take a bit of time to work out, but I can help if you need to.
This would have a few downfalls though, for instance if the rate changes then it would change for all past invoices. This can be fixed by having a valid beginning and ending date for the currency table, and check that against the invoice date.