If you read the NATO charter, in particular Article 5, it makes pretty clear that NATO is a defensive military alliance.
And as long as it is defensive in nature, then it ipso facto can't be imperialist, since there is nothing imperialist about self-defense.
This reasoning doesn't make any sense. The Warsaw Pact was also avowedly defensive in nature, and of course the only countries it ever invaded were its own members. It would be absurd to allege that the Warsaw Pact was not imperialist because its charter made it clear that it was a defensive military alliance.
The most obvious parallel to NATO is, of course, the Delian League, a grouping of Athens and its allied states. This was also an association of self-defense, and as it happens the term "Delian League" is basically equivalent to "Athenian Empire". It may be that NATO is not an imperialist project (and in fact I am not claiming here that it is), but that argument would have to actually be made rather than ducking behind definitions and making vague associations.