The simple answer is yes, but if you want her animation and sound to match, then that's a whole other story.
The demoness info is in DEMONESS.ATOM (in DATA.KFS) and then you can just change her movement type from 0 to 1 (float - like beholders or sprites) or 2 (fly like dragons or girffins). Note that Ghosts are a movement type of -2 (allowing them to move through obstacles).
Now if you want to make their animations and sounds match, then that is a whole new story, but the animation that makes her fly is "avoid".
Note that in my H3B mod, I made Demonesses and Archdemons flit when Giants do their earthquake so that they avoid damage just like dragons and griffins since they both have wings to avoid it. The Archdemon's "fly" animation is "special".
If you look at the animations {} section in the ATOM, you'll see there are at least a couple dozen animations. To change how a unit moves, you'd at least need to change the move animation (this is the move= line and is usually the unit's name followed by an _move.bsa - the demoness' is demoness_move.bsa (BSA files are located in ANIMATION.KFS)); however, it is usually more complex than that - if you check out the dragon's movement, they have a takeoff, flight, and land series of animations.
I don't know what edits *.BSA files, maybe someone will post that information here.
Then there is also the sound. Pretty much all the sounds are WAV files and they are located in SOUNDS.KFS. When the Demoness moves, her heels make a sound on the ground, so you'd have to fiddle with that, too. Sounds are in the attachments {} section of the ATOM and then a numbered section that has typically several sound{} sections. Each sound{} then identifies an animation the sound goes with and the file= line identifies the wave file to play when performing that animation. For Demoness (you guessed it!) it is demoness_move (the .wav is omitted). You can change it to point to another WAV file (i.e. demoness_fly) or add a different WAV of the same name to the mods folder to make it sound differently. Note that you may need to change other sounds if they don't match.
Good luck if you want to go change them to fly around in combat with proper animations and sounds - let us know how it goes!
/C\/C\