There is no adjectival form of déjà vu: the phrase literally means "already seen", with vu being the past particple of the verb voir, which means "to see". If you substituted the English, the correct way to incorporate it into a sentence would be "an experience already seen" (i.e., "an already seened experience" would be incorrect).
If you wanted to use it before the noun, you could rely on the listener/reader accommodating the seemingly incorrect usage of déjà vu (i.e., "a déjà vu experience") or a neologistic formation (e.g., "a dejavued experience" or "a déjà-vued experience"), but it's probably going to be clearer if you use a different adjective that conveys the same meaning as the phrase.
To this end, the word you're likely looking for is uncanny. A usage attributed to Sigmund Freud, he speaks of it thusly in "The Uncanny" (PDF):
The subject of the “uncanny” is a province of this kind. It undoubtedly belongs to all that is terrible—to all that arouses dread and creeping horror; it is equally certain, too, that the word is not always used in a clearly definable sense, so that it tends to coincide with whatever excites dread. [...] I will say at once that both courses lead to the same result: the “uncanny” is that class of the terrifying which leads back to something long known to us, once very familiar.