Are your tables connected in the DB? You can create relationships in the main connection in a database bound InfoPath form - submits to one-to-many data connections are supported in InfoPath filler. Yes, if you must add code, you can - and then you can pretty much do as you want. If you decide to use code, you'd probably want to add a repeating table to your main data source, use code to populate the data when the user selects the dropdown value, then use code to write changes back to the DB when the user submits. Again, I'm assuming you are planning on an InfoPath filler form, not browser.
Hilary Stoupa
