wow, and btw that's not a "counts as army" that would just be proxying -- even with a fluff story.
Proxy armies are basically exactly what you are describing: Using one army to represent another with little or no modification.
"Counts-as" armies require a bit more work and typically involve conversions, painting, clearly represented units and something I like to call "effort" -- Kevin's Night Lord's army is a good example. Most of the pre-heresy era armies are good examples.
Saying your necron models are space wolves is just proxying models. which is fine in friendly games as long as you are cool with it. But don't try to polish a turd and say it's a diamond. or try to pass off your fanfic as cannon. c'mon man.