At the moment, no it's not possible. Money is not an "item" you carry around
as as such. It's just an array of numbers saved in the player. Keep would have
to be severely modified to accomodate that feature. Also, paying with your
lowest-value coins by default is considered a good thing by most people.
You can save your money in a purse or you can just go kill a few bunnies in
diggas and voila - you have thousands of mowgles again.