Is it possible to import a new variable data field into an existing database

Is it possible to import a new variable data field into an existing database

The Mass Update function only seems to allow us to import a single piece of static data to an existing database so is there some way of being able to append other data to an existing database?
I have a customer who wants to send me an individual coupon code generated by them to add as a custom merge field for every record on our database.
Is it possible to add this somehow please?