Yeah, probably the smartest solution.
I changed a few things as described here
http://forums.opengoo.org/index.php?topic=1495.0.
That way you can have different currencies for different billing categories.
It is, however, not a very wise way to do it, firstly when adding up the bills, you need to GROUP BY currency, secondly it would be necessary to have a new table with the currencies to avoid redundancy, to avoid people writing the same currency differently, ie DKr, DKR, Kr, kr, dkr, etc.
Maybe a thought for the future, but having one central currency makes a lot of sense.