have a look to the user guide where the interactive example forms are also available
the ‘Passing Data to Forms by Querystring Example’ show show you can populate form fields (it isn’t the ExternalCall function you have seen – this explanation, and a few others are still being worked on for an update to the user guide)
and also there is ‘Allowing a Form User to Save Then Submit Later’ to show how you can have users saving and completing later.
in respect of ‘code after submit’ … yes, but not in that exact manner. any ‘form’ actions (initiate a print, write to a database etc) are best handled before the actual ‘submit’, and then use the workflow functions to handle the notification to your users (as you look at the ‘save and return’ form example you will see how the notifications are best handled, as well as in the user guide in regard to how to implement workflow
ps to intergrate to other databases you will need the Connection Agent modules. Talk to your account representative for more information on this