If you want iron to simply be items located around the map (or respawning at a certain point) and have an integer variable to keep track of them, you could use an inventory unit to gather them.
The trickiest part is to have units/upgrades/buildings require a certain amount of iron in addition to its gold/lumber costs. The only method I can think of is using a "dummy" upgrade with 100 levels called "iron amount", and have things require "iron amount" to be at a certain tech level before it's even allowed to be built. Upon building/training the thing, the tech level of "iron amount" is reduced by X.
If your question was, "Can I make it work like Gold and Lumber Do", the answer is probably no.
Another method is to treat mana on particular units like iron (because you can have a mana cost on certain things). However, I think it's impossible to have a gold/lumber cost on spells (except for the goblin hut reveal spell).