Sure you can...(as long as your system will handle it)
I'm not so sure I agree with Kirby here. If that was the case, you'd be forcing the user to calculate each foreign currency transaction by hand/outside the system (which increases the chance of error) and I would think you'd want to minimize things of that nature.
For foreign currency transactions, simply tell the system what currency this transaction is, input the amount invoiced (or whatever) and let the system convert it back to the base currency of the "company" that you are in.
By definition, this means that someone, on a timely/periodic basis, must go into the system and update changes in the exchange rate, but that's far easier than having your staff attempt to input numerous transactions and make that calculation by hand...
In other words, let the system do the heavy lifting and make it as easy as possible for your staff