The paralysis is easy: just make a self-targeting interaction with a longish cooldown, which applies a short-lived paralysis syndrome.
To create an item, you'll want to make a material emission - see the dwarf or magma crab raws for examples, and modify them to be self-targeting. You may have to play around with the properties of the material in question to keep it from harming the critter, as the material emission is usually launched at high velocity at a different target, and I'm not sure how it would react when the target is the source...
Or if you don't mind DFHackery, there's a plugin that can automatically detect when a syndrome is applied and spawn an object or creature into the game at the location of the affected creature. Dirst(?)'s mod The Earth Strikes Back is a good place to look for inspiration on how to use that.
Although I would probably go with Eric's solution, what you are describing Button is how The Earth Strikes Back worked in its early versions. Version 1.22 is still available in the Dropbox folder where I keep all of my old versions. Since 1.30, it watches for job completion events, and spawns a creature or item at appropriate times. Salmuek, if you want to see that in action, look to the current version of the mod.
You could trigger the molting to a sleep or worship "job" if that's not too often. With some nontrivial effort, you could maintain a counter for each creature and spawn a shell after, say, 4 sleep "jobs."
Edit: and Modest Mod is awesome
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Topic: [MODDING] 0.43.x QUESTIONS THREAD (Read 201672 times)