Zombie Shell is a skill to limit the number of takeover bot a player can use in
combat. 

Zombie Shell have core limit. When the sum of core of all takeover reach the
core limit, all new takeover bots are shutdown. You can shut down a bot to turn
on another bot.

Another name: Zombie controller, Zombie host, Zombie shell. Zombie-net

Ah, I understood it now.
It's a known fact that if the player haves too many bots, the game may crash.
So this skil would limit the maximum number of droids Tux may have, by counting
droids on faction 'self' upon takeover, and if it exceeds the limit (you have
too many droids), it hcf.py the hacked droid and shows a message.

Every level of this skill allow you to control, say, 25 bots. You start with
Level 1, but how can you improve it's level? Shopping, or is it a gift when you
advance the story?

Well, if you think it's a good idea, count how many droids are on faction self
and you can already do a patch for this -- if you still have interest on this
idea anyway.