How about a dummy unit that will cast the Inner Fire spell on your unit??
Create an invisible dummy unit and a manipulated ability of Inner Fire(Human Priest ability) that increases only armor.
Then, create a spell (I suggest Thunder Clap) and put all its values in that way so they wont do a thing. Set for example damage 0.00, duration 0.01 e.t.c.
Just always rememer to be a spell that is casted immediately and not like Storm Bolt or Frost Nova.
Finnaly, via triggers order the dummy unit to cast the Manipulated Inner Fire spell to your hero when he casts the Manipulated Thunder Clap.
P.S Set the Durration of Manipulated Inner Fire to the time you want your hero to have the armor bonus.