So, you mean it's a series with multiple books, with only the first one or first part completed?
In that case, I do believe it's fine to pitch. I'm not sure if the submission says 80k min to pitch, or to publish. Usually they want the novel to be completed, or mostly complete before pitching.
But I'm not sure how it works it your novel is technically complete, but under 80k words, with more books in the series to come right after it.
You could probably pitch the book as a whole series, or like you said, multiple arcs, with the first part under 80k, but still completed. That way they know it will be over 80k as a whole and at least the first full portion is complete.
(not sure what the final say will be, of course)
If you're saying that you have 80k words done but that's only part of the book and it has plenty of books/arcs to go, I think I'd say the same thing. Pitch it and find out.