Echoing bradrn, the thing you have full control over as a creator is the conlang's data; you get to say what statements are grammatical, and what statements are ungrammatical, and in which contexts and for what purposes you might say a statement.
You can't declare what the best analysis of some part of the language is by authorial fiat, though. If there's some analysis you want to fit best, that's fine, but you have to accomplish that by tweaking the data so that your desired analysis is actually the best fit—otherwise it doesn't mean much.
You can't snap a part of speech into existence with your godlike authorial reality powers, but you can snap a bunch of words and syntactic structures into existence such that the best analysis someone can give for them is to posit a particular part of speech.