The term "hacky sack" is not a verb. It is, as you say, a noun. But just as people have been known to "hula hoop" or "pogo," you could coin "hacky sack" as a verb. People would understand if you were to ask, "Wanna hacky sack with us?"
Yes, it could be used as an intransitive verb, such as "We hackey-sacked together for four hours yesterday." However, it sounds strange as a transitive verb, as in, "We hackey-sacked the ball around for four hours".
