I don't think it'll be impossible, but a few more things need to be in place before it'll be reasonable, I think. As it stands now it'd be difficult but doable... I'm almost tempted to do it just to see if I can!
For one, you can make changes to the files the program outputs as long as you run the TestBat.bat program externally (and hold onto your changes in another file so that when you DO run that command to update the files you can restore it quickly).
Just as an example, though, in the code the editor creates you have npcText01.dat and npcText02.dat, each of which hold 256 text entries as you set in the text area.
In its simplest implementation, not using the overworld/underworld extra text box, I'd use Text506 through Text511 to store the information.
If the screen ID were between #$00 and #$55 I'd look up Text506, if it's between #$56 and #$AA I'd use the last byte of Text506 and 254 bytes of Text507, and if it's between #$AB and #$FF I'd look at the last 2 bytes of Text507 and all of Text508. If it's an underworld screen I'd do the same thing but with Text509, Text510 and Text511.
Of course, none of this takes into account each warp having its own warp in point. If you want each one to have its own warp in you'd need another 6 text fields worth of data.
So it'd be hard, but not impossible.
Basically, to add a feature we sacrifice another -- text space.