Hi - the only way I know to do this with a form that is not based off the Access database is with custom code, unfortunately. You can add fields to a database bound form, as long as you add them to the root node - but that doesn't help you with your existing forms that are probably already filled out.
If the data can be moved to SQL, the company I work for sells a web service makes submitting data from an existing form to a database much simpler.
I wonder - if the fields are promoted properties in the library, perhaps you could do something with a custom workflow - write the values to a list, and link another table to the list... then have some other logic that updates the DB ...