sorry for the double post but to bring your attention Im double posting:
hp are extremly easy, there is item:
Item Permanent Life Gain (+50)
for damage:
Item Permanent Damage Gain
the armor is little bit more tricky however
You have to copy something like item armor bonus (+5), remove the icon(Item abilities with no icons have no icon, Unit/Hero will show green square) and give it like 100 levels(more then 100 lvls will make the map load for ages) and change the values to whatever you want.
Right now Ive tested it and you cant add decimal values only integer(1,2,3 etc). If you want even more, copy that as well and enter higher values.
Every second(or how much you want) increase level of the item ability with armor by 1 and give the unit one Item Permanent Life Gain and Damage Gain.
There ya go, it should work just fine
edit:
actually, the Item Permanent Gain are abilities, you can create a power-up item and you give those to unit(base it on like tome of experience)