The easiest way is to use Pick All Units in a Unit Group and Order Picked unit to Attack-Move. You may need to reissue the attack order once in a while, if the zombies have abilities (or alternatively, the zombies may refrain from using the abilities). The pick units in a unit group works for 12 units max. Therefore, you may need to separate larger groups into smaller unit groups.
If the zombies keep returning from where they came, try recycling their guard positions, deleting all "start AI triggers" (especially melee game - start ais), and changing the zombies to other than neutral hostile (or alternatively increasing creep return range and time).