David,
This can be done of course but will not be all that easy for a non-developer. It doesn't sound like it would be very user friendly either. The issue is that there is logic coded into the eSearch that will automatically switch the text input box to a combobox if a domain or userlist is specified, so you would have to avoid breaking that code. If you look at the mxml code for the searchWidget you can find the textInput group that is where you would begin adding your extra text input boxes. Best of luck