Of course you can. The fact that we can talk about what the implications would be if God didn't exist shows that we can divorce the concept of God from necessary existence.The premise has an explanation, long and short ones, but the shortest version is conceptually you can't divorce necessity and hence existence from the concept.
Of course it's sneaking. "Necessary" in this context just means "must exist." Arguing that God exists because he's "necessary" is effectively saying "God must exist, therefore God must exist." You're begging the question; you've just got a lot of extra hand-waving going on to distract from it.It's not about sneaking, if God is the Necessary being which he is and only he is, then that attribute/description proves he exists.
Which is nonsense, of course.What makes us know God is the Necessary being, this has been already shown, the ontological arguments of the past due it by greatest and perfection, and I do it, by sheer size.