Currently I can't do them using the ruleset.
Right now my Brainsucker Launcher is a “Zombifying Cannon”, and the actual Brainsucker unit is bound to do the same. Now: the cannon is actually cool and fun to face, but the “zombifiying brainsucker” is a just another chryssalid.
Nevermind the launcher. But it would be useful to have an ability that puts under permanent alien control the target hit by the attack.
The same ability could be used to make things like the “black goo” from that first XCOM (the FPS) trailer.
I guess this is another “please do the thing I need”-kind of topic. I don't like to make them and I apologize, but there's not much else I can do when it comes to making aliens. While there's a lot of options for new weapons, for new aliens, save for a couple of tricks, you're mostly reskinning the old ones. The players got their toys, shotguns and so on; now it's time to give some new toys to the baddies!