Well, they aren't msg files so it won't work. Luckily IN's ending files are pretty much plaintext and some script instructions. It shouldn't be too hard to figure out what things do.
@mbgm sets the midi file for midi mode.
@bdata sets the background image.
@c sets the color of text in RGB form, as an integer. (@c15790320 = 0xF0F0F0 = (240,240,240))
I'm not sure of the difference between @r and @w but they both make new dialogue panes.
This is only after a cursory look. Shouldn't be a problem, especially if you're only modifying text.