mutisite installation with different domain/urls

I am working on setting up a multisite installation for a university.

We’ll need to host multiple sites like:
sitea.exampleu.edu
siteb.exampleu.edu

but I would like to have an independent top level domain like
wordpress.exampleu.edu

(exampleu.edu is not WordPress so I can’t use it.)

How would I do this as opposed to having to do something like
wordpress.exampleu.edu/sitea