Toggling the "Can attack air units" flag will allow a ground unit to attack an air unit, but will
not give it the scrambling bonus. That is, the air defense of the AA unit is not quadrupled unless it, too, is an air unit. Keep this in mind when setting your attack and defense strengths - an AA unit should still be vulnerable to land units.

Make your AA unit an air unit, and give most or all land units the "can attack air unit" flag. Keep fighters' range the same, they will be unaffected. The AA can then be attacked by land units, and it will also gain the special defense bonus against fighters and bombers.