Hi,
Honestly if both fields are not meant to be mapped in SAP standard delivery, you shouldn't try to map them... and there is something very easy that you can do instead in order to achieve your requirement: follow the cookbook available at http://service.sap.com/utilities to add customer fields to CRM business agreements. With a little bit of luck appending one field to 2 or 3 structures will do the trick. We used it to replicate the dunning procedure to CRM and that's working great!
Regards
Nicolas.