Giving hero abilities to units through unit upgrades

How do i say make an upgrade give footmen an aura that adds to their defense and possibly change the defense type in the form of an aura that adds defense to other footmen/units?

The only way to change the defense type of a unit is to morph it to another unit: for example, look at troll berserker and siege engine upgrades which morph the unit type from one to another, the upgrades enable a "chaos" ability (so named because it was originally used to morph orcs into chaos orcs in the campaign) that morphs the units into a new unit type. The new unit type can also have the bonus armour aura which you can supplement with an ability based on the item armour bonus to give a higher armour bonus to the aura unit (or you can simply give it higher base armour).