Lets assume your other table is called "otheritems"
Fork the controller "otheritem" (create a "fork" folder in the admin part and a subfolder "controllers", copy and paste "otheritem" from the _fork/controllers folder)
Open the "otheritem" controller from the generate code, copy and paste the public function save from it in your forked controller.
In the top of the function there will probably something like:
$result = parent::save();
You can do a check if the result was a succes and then add code to save an entry in your feed table.
If you do not know how to do that, then I can provide futher example code