It's kinda of boring, but can be done.
First, you need to see if the unit has a flying animation. If it has, you can go on.
To make this work, you need to create two units (Ground version and Air version), and a morph ability.
Some units has the Flying animation with the "alternate" in addiction. E.g.: "Stand alternate". In the flying unit, change the "Art - Required Animation Names" and the "Art - Required Bone Names" to "alternate". And also change the "Moviment - Height" and "Moviment - Height Minimum" to whatever you'd like. And be sure to set the "Moviment - Type" to "Fly".
In the Ground unit, be sure to set the "Moviment - Height" and "Moviment - Height Minimum" to "0.00", and set the "Moviment - Type" to "Foot" (Or "Horse", according to the model).
The ablity, you could create a Custom Ability using the Gargoyle's Stone Form, to make it easier. Set the "Data - Alternate Form Unit" to the Flying model, and the "Data - Normal Form Unit" to the Ground model. Also, in the "Data - Morphing Flags", check the "Immediate Take Off" and the "Uninterruptable". And give this ability to both units.
If it doesn't work, fell free to VM me.