Currency translation

We want to post document in company code currency USD and want to translate it in foreign currency for eg, GBP. How can that be achieved? Tried to change the currency in General Ledger Master data .to GBP but the system wont let me post a document in USD because the Gl account is assigned to GBP in master data.

How can we post a document in USD and have the system translate to other currencies. FYI, the company code currency and the group currency is USD.