плацдарм(something that allows you to make progress or begin to achieve something: a stepping stone to a more lucrative career. OALD Alexander Demidov)
переходный этап(пример: Others, typically economists, respond by arguing that so-called sweatshops are simply a stepping stone on the path of economic and eventual human development. A.Rezvov)
средство для достижения цели(тж. stepping stones: Do you view your colleagues as stepping-stones? – Вы рассматриваете своих коллег только как средство достижения цели? • This job is a stepping stone to better things • Education is a stepping stone to a good job • Modelling is seen as a stepping stone to a career in moviesTaras)