I realise this is a very general question, and I am not asking for specific instructions on how to implement this (although if you have a link I would love to see it.
I often have a scenario where I have two related tables. And it would be nice to add the possible related records via ajax on the same form when I am creating the ‘parent’ record.
First one, is to have a compobox to select external values. And near the combobox, you could have a sort of button that open a modal dialog with a form that permit users to add new records.
Second one, is to leave free the field, and use autocomplete jquery plugin.