Well, you can command a creature to try to fall asleep, for sure. A creature who's suggested to fall asleep will probably curl up into a little ball or go to a bed and pull covers over them and close their eyes and try to fall asleep. Just like if you ask a creature to write a letter, they will go find an inkpen and some paper first.
That being said, a Frenzied Berserker probably can't fall asleep while in a frenzy, or in fact do anything but continue to frenzy. You can suggest he stop frenzying, but in all likelihood he's already trying to do so but is unlikely to be capable of it.
Realistically, the best way to deal with a Frenzied Berserker is to just have your cleric prepare a couple Calm Emotions or something. I think of it more as an NPC class than anything though.