way how to add to unit more than +1 armor point through upgrade is to mod armour_ug_levels stats.

Exaample: Peasant Crosbowmen have armour_ug_levels 0, 1, 2. So they will recieve +1 with leather upgrade and +1 with light chain upgrade. If you change that to: armour_ug_levels 0, 1, 1, 2 they will recieve +2 with leather upgrade and +1 with light chain upgrade.
armour_ug_levels 0, 1, 1, 1 means that with leather upgrade you will recieve +3.

Catch is,that every unit can recieve only +3 upgrade, so it is not possible to rise armor to higher levels than +3.

PS: ofcourse you cant forget to add correct upgrade skin to armour_ug_models, so for +2leather and +1 chain it will look:

armour_ug_levels 0, 1, 1, 2
armour_ug_models Peasant_Crossbowmen, Peasant_Crossbowmen_ug1, Peasant_Crossbowmen_ug1, Peasant_Crossbowmen_ug2