Customize Bookkeeping Transactions Report: Print custom field option value instead of name/label
I assume you're not asking how to customize reports in general, but what customization you can make to get your desired results.

You can use the alterDisplay() function to modify the output of your report.  Check out CRM_Core_PseudoConstact and its methods (getKey, getLabel, getName) one of those is likely to do it for you.