you generally cant, the chances are she will peck at them and more than likely kill them! better to stick them either under a heat lamp or electric hen, harden off when fully feathered about 6 weeks. we dont introduce chicks to adult birds till they are about 3 months old.... and then its done with care.
The only way it is sometimes possible is if the hen is broody then you slip the freshy hatched chicks underneath her, but even then i wouldnt risk it