[Mobhack]

Or method b)

Make a formation for your own use with a truck as unit 1 (the game determines formation type by what is in slot 1)
Put planes in after it, or platoons of planes.

You can now buy planes in any battle regardless of air strike points, including as campaign core (but the upgrade code wont work as planes are not core items. delete, losing any experience and re-purchase is about the only way to do so).

Anyway - that was what I did in the editor I made for SP1 back in the day, should still work. It is a cheat, but may be useful for own use. Not for any scenario or campaign though as end users won't read any text you supply and will come on here whining about not being able to upgrade planes etc.