A slightly different approach:
Create Zombie. Zombie starts by himself, and picks up to 9 pals. When 9th pal is feeling allright, Zombie cast split, thus killing himself, and creating Zombie and Zombie, each of them with 4 pals. Continue. (This is what you described)
This can be done this way-
Zombie is a unit that can grow up to 10 (the numbers can be changed however you want).
Zombie raises a pal (see above) every time he kills.
Every turn, Zombie restores 20% of his hp (so every 5 turns a pal joins Zombie).
When Zombie's total health reaches 10 times Zombie's health (or, if you wish, full health), zombie can use an ability that costs 100% of the stacks health, summoning 2 new units, and killing him in the process.
Another way to do this is, AI will to cast Summon Zombie is [Zombie's health]/[Zombie's total health], the spell costs the health of one pal, and summons Zombie without pals, or a similar fashion.
Hope this helps.