This is probably quite a basic question.
I have a vendor file with multiple currencies going to a single account. Can anyone advise the best way to do this as there is no option in the Data Source ...
Flow is good option when working with Transactional Currency.
If you set up some Currency Input accounts in the Flow and set the "Flow Processing Type" to "Is Alternate Input Currency For All Accounts" and then create some Translated Members and set the "Flow Processing Type" to "Translated Using Alternate Input Currency, Derive Local" selecting the Input member you created as the Source Member.
You would likely only want to aggregate your Translated Members into the Flow dimension so you don't duplicate.
If you have access the new material on Academy, "Designing a OneStream Academy" (from which the screenshot above is taken) there is a good video explaining this in the context of building a planning application where Legal Entity is not in the Entity dimension for a planning Cube but rather the Department. It therefore becomes neccessary to have some means of entering in multiple currencies.