You edit 
misc/main_sanity_events.cfg
Then you call your custom insanity event with 
StartRandomInsanityEvent();
Being that it's at random which 
Insanity Event is called, you wanna disable all the others first.
SetInsanitySetEnabled( asInsanitySet, true/false );
So, basically if you have your custom 
insanity event and it's 
set name is "
Frost" (it uses sets, not specific names)
You gotta use 
SetInsanitySetEnabled(set, false) on every other set in the file before you call 
StartRandomInsanityEvent();
It's supposed to work %100 but still there's a weird chance, like %15 of the time, your 
StartRandomInsanityEvent call gets ignored. As if the game decided that none of the insanity events are gonna play.
More info
Here's the guide dnAlange did once.
I didn't read it, but we talked many times and tried many methods and still the function fails time to time.