Using Wild Morph? Sure.
I can only assume what you're really asking is can a Wild Order druid employing Wild Shape use Knockdown. I haven't scoured every Form spell's entry, but if they all have the bullet point "-One or more unarmed melee attacks specific to the battle form you choose, which are the only attacks you can use." (emphasis mine) then I would say no. Knockdown doesn't have the attack trait, but it directs the user to make an Athletics check to Trip, which does.