No thank you for the plugin!
Regarding external DB, certainly aware that you folks can't possibly know which external applications folks might want to push out to which is why I can understand it not being a part of the default plugin but possibly could be a paid add-on for folks who can't code a hook themselves. Which is part of the whole sell for WP, simplicity.
Scenario to better illustrate the meaning:
John Doe has a form on an old vanilla HTML site that used php to scrape and send all submitted info to a SQL database. John moves to WP, and sees the impressive Ninja Forms plugin which would would work great for his form that he wants to transition. The catch though is that John already has a DB where the data is collected (and accessed by other parties for information retrieval), external to WP, and would like to setup the same fields in Ninja Forms to point to the old database.
John is a novice coder and while he can understand and tweak code, is by no means proficient.
A ninja-form add-on that allows this functionality would make Mr. Doe 5-star happy :)
Hope that makes sense. THANKS again for all the hard work you folks put in to make this plugin. Truly appreciated.