There is no zombie villager spawn egg in MCPE, despite zombie villagers internally being a distinct and separate entity from regular zombies, like husks. In the Java Edition, there is a zombie villager spawn egg, which is oddly absent from MCPE. The zombie spawn egg never spawns zombie villagers, and so currently the only way to get one is either to use the /summon command or to find a naturally-spawned one or one created from a villager being infected.