The Armor bonus is added to the Warframe's total armor, and does not increase the Warframe's base armor.
Mods like Steel Fiber that multiply a Warframe's base armor do not increase the effects of Health Conversion, while those that multiply total armor, like Ironclad Charge, do.
Warframe abilities like Iron Skin, Warding Halo, and Snow Globe use the following expression when accounting for Health Conversion: Modified Health = (Base Health + Ability Modifier × ((Health Conversion × # of Stacks) + Warframe Base Armor × (1 + Base Armor Bonus))) × (1 + Ability Strength) + Absorbed Damage. For example, a Warding Halo with 3 Stacks of Health Conversion and a maxed Intensify would yield (900 + 2.5 × ((450 × 3) + 175 × (1+0))) x (1+0.3) = 6126.25 health before damage absorption.
The armor added is the value shown per stack. So with a max ranked mod, the armor added would be 450 for the first stack, then 900 total with the second stack, and 1350 total with the third stack. This is as per latest patch notes and multiple comment confirmations below.