Slavery is not allowed under libertarian ideals, because no one can own you but you. However, you can sell yourself, lease your time, or even kill yourself; because the fundamental concept of ownership is the right to destroy, not to utilize.
Right, you can sell or donate yourself into slavery but you can't be enslaved against your will.
Military conscription is slavery, volunteering to serve is not. So yes, you can actually accept the dominion of a "superior" by voluntary contract.