sounds like you would need to write either a custom validator and or some custom handling of the form - the SQLFORM() helper assumes that what is the output of the validated field is exactly what you want to store in the DB, but it sounds like you want the output of the validated field to be made of data that was both submitted by the user and also by what already exists in the DB.
look for custom validators in the book and see if you can build one that will work for you. sounds like you have defined the requirements so that's a good first step!
