Custom currency field converting another field's value

I have an opportunity record with an estimated value field. I wish to add another currency field which shows this same estimated value, but converted into another currency -- and not the system currency nor the user's currency.

estimatedvalue <- currency is GBP because of user settings

new_customestimed <- currency being USD, regardless of system default currency or user currency

Is there any way to set transactioncurrencyid for individual fields? What's the solution here?

Thanks