I don't know, I have to disagree with justADude, and while he won't like the reasons why, I can't help but think them logical.
A spirit's fundamental form is as an immaterial entity. In this form, it does not have ITNW, as clearly stated by RAW quotations provided by JaD. In order to possess that power, it must use another power, Materialization. Now, sure, you could interpret the rules to mean that powers provided by powers are able to be endowed, but that sort of opens a can of worms that all make very little sense to me, and the rules are abundantly clear at every turn that while spirits may possess the power, the power is provided by materialization. It is definitively interpretation to say either point of view beyond that is correct, given that the rules say nothing at all about endowing powers provided by powers, and repeatedly quoting that when materialized, the spirit possess the ITNW power does not change the fact. I won't bother saying this is merely my opinion - if you can't figure that out, you've no place responding.
If you prefer to stick to your "the literal wording and nothing else" approach, then one could say that if a Spirit takes the form of say, form fitting body armour, since it mimics that object in form and function exactly, it would only be a set of form fitting armour. No added armour rating from being a spirit, no ITNW (unless endowed), no perks at all except the exact ones provided by Form Fitting Armour (6B/2I). An effect much more cheaply provided by some under two thousand nuyen and under twelve availability items.