Rexxar campaign has those on main maps for acts 1 and 2 (but it uses starting regions, as it tracks groups). There you'll also find that it does not allow to spawn where player can see them, if you want.
How I would do it (from memory and by basic logic, sorry for not using trigger formatting and using pseudocode):
Trigger 1 - track your creeps
Event - Map Init (or run after creating them)
Actions:
Pick each unit owned by Neutral Hostile
Set TrackedCreep[index] = picked unit
Set CreepPoint[index] = Position of picked unit
Set index = index+1
Set TrackedCreepAmount = index
Trigger 2 - basic respawning
Event - Every 60 seconds
Condition - InCinematic Equal False
Actions:
For each index from 1 to TrackedCreepAmount do
If TrackedCreep[index] is dead
then Create 1 Unit of type (unit type of TrackedCreep[index]) at CreepPoint[index] facing random angle
Main point - store their starting points if you want to spawn them at starting positions.
Note - not sure if unit type is tracked for unit that has been removed from game may have to track it too (in trigger 1 Set TrackedType[index] = Unit type of picked unit)
