Excel will use the numbering format set in Windows for currency.
There is no way to get Excel to work with multiple currency formats while keeping them as numbers.
You can format a text string to "look" like a number with a different currency, but Excel will be seeing it as a text string, and not a number.
I have had MANY questions/cases submitted to my position on the Frontier Team from EMEA regarding this exact question.
My advise is to keep currency designations out of the scenario until the very end.
You can keep a seperate table for conversion rates (euro vs. dollar) and use column headers to indicate whether or not that column is in Euros or Dollars.
In the end, if your system is set to euros, and you need to display in dollars, you can take your final dollar calculation value and add the $ via CONCATENATE().