When french are using an english word, they first use it like english would do, then they try to make it french. Generally by adding -er after the verb to make it a first group verb and make it easy for everyone.

For example with “stop” (un stop, stoppeur, stoppeuse, stopper) or “bluff” (un bluff, bluffeur, bluffeuse, bluffer). We can see that it feel natural for “slam” to be “un slam“, “slameur”, slameuse” and “slamer“. I think those are now accepted french words because the link I provided is “Larousse” a very famous french dictionnary.

Now, “slam” as an adjectif (“poésie slam”) is not in the Larousse french dictionnary, rigth now. It seem to me that there would be very few use case for such an adjective (“texte slam”, “rythme slam”, “diction slam” ?). Moreother, I think that “poésie slam” is a pleonasme. If it’s “un slam” it’s already poetry, and you could simply use “slam” instead.