The treasurer for our club uses excel to manage the accounts. It would be very easy to write a macro to produce a csv in the correct format - if DTBilling would allow importing of csv files as well as exporting!
I take it exporting just reads the DTBilling DB to a csv file. It should be fairly easy to do this backwards, updating the db with new info. Presumabley there is a row for each registered user, so each one can be updated. If there wasnt a row for a user, just get it to create one.
It would need to check for this, to prevent you getting multiple entries for each user.