I have a script spell I want to add to most races, but the CK will only allow me to add it to certain races.

Nords, Imperials, etc., work. But the Elder, Dremora, and Fox races, etc. do not. I can even add the spell to horses and mammoths.

Am I missing something simple? Thank you

Those latter ones aren't playable races- they're creatures.

Horses and mammoths aren't playable, but they allow me to add the spell. So I'm quite confused

You still trying to rig up that on-hit script? Send me a PM, we can write up a SkyProc patch for it if you want. It'd require using a patch, but it could be done simply and easily to all NPC's (and that includes attaching scripts, working scripts, to those that aren't allowed in the CK).