I believe the original derivation is Ancent Greek via Medean Aramaic. Thus, the strictly correct form would be, qualoboloi, qualoboses.
However it depends on whether you are talking about the qualifying tournament as a whole, or individual or a plural of matches in said tournament.
In the former case, it should take a gerundative nominative form: qualius, qualia (more than one qualifying tournament)
In the latter case, the logical thing to do is to take a reflexive intransitive approach, and default to: qualiandra, qualiesma
There's always a nerd to bore you with semantics Hope that helps.
Spoiler
Everything in this post, outside of this spoiler, is utter nonsense; hogswash of the finest degree
I recall in Roman times, the qualifying events for the gladiatorial games were of course the ultimate test - only one winner ultimately survived. And then they had to play or fight seven more times to win. The phrase Grand Slam came from this originally - to be slammed was of course what it means, but to win the whole thing meant you had not only slammed but grand slammed. To lose and hence die in the qualifying rounds has got lost but originally was termed "to be qualied" - or to lose in the qualifiers