You can't change a unit's roll in the air. You can fake it through a couple of means though.
(1) You might be able to use a modified impale model to do it. Impale is the only spell that attaches a unit model to the effect model, so if you make a dummy model that flips and use impale as a base, perhaps it would work? It is a decent amount of work though, and requires knowledge of model editing. And I'm not even sure if it would work. I just remember Jesus4Lyf mentioning it as a possibility in the past.
(2) Use a series of dummies with the unit's model set at different roll angles. Each version of the unit model will act as a "frame", similar to a flip book. You will create all the dummies at once, play the appropriate animation, and just show/hide the appropriate model for each "frame" of the spell.
Note that (2) is a bit limited in use since you would have to make a bunch of objects for every different model that may cast the spell. But if you have only one type of unit who will cast the spell, it is a pretty good option. It doesn't look too bad if you make enough units. Credits to Tinki3 for the original idea--he submitted it for a spell contest 8 or 9 years ago. It was pretty smart--I'm not sure if he came up with it, but that is where I got it from.