That depends on exactly when and why you want this state to trigger. Some rapers have the data for this built into their own code; the victim just needs to have the correct animations for it. gettag's
Brian and
Tower of Bloodyfight characters, and most versions of
Horuakuti's mating press Kuromaru, have this functionality. You should be able to find the animation numbers in a victim that already has the proper compatibility, copy it over, and tweak as required to get the result you're looking for.
For Brian or TOBF, most of gettag's victims and some of fapmaster's edits have the compatibility you'd want to replicate, though you may need to look through multiple victims to find all of the animation numbers.
For mating press Kuromaru, if you don't mind working with an older version then mugeryona1762 should have... version 6, it says in my notes? of Horuakuti's Kuromaru, and a compatible character.* This version doesn't have the side missionary position, but does have code for normal, sleep/hypno gas and impregnated/post-KO victim animations.
*Lolicon warning on the compatible character. Eko from Vanguard Princess, if you want to look her up before downloading.
For newer press Kuromaru,
@derakino999 posted a MEGA link recently that contains several characters which - while I haven't tested them - look to be among the most recent releases of mating press Kuromaru and compatible victims. I don't recall the differences between one version and another off the top of my head, but if you're willing to go digging I've made several other posts in this thread with that kind of information. Forum search is down currently, but you should be able to get some results in a general internet search engine using the terms:
Code:
site:https://ulmf.org/ "Tigris" "kuromaru"
If you're trying to get ahegao states with rapers that
don't have code for it, that's probably going to be more complicated. I don't know much about how M.U.G.E.N. code actually works so I'm just throwing out ideas here, but I imagine it might be possible to set up something in a .cns file that a) watches for a specific set of states (the "standard" victim animations) to trigger and increments a counter when they do, b) activates some kind of flag when that counter reaches or exceeds a certain number, and c) when the flag and trigger state are both active, overrides the trigger state with a custom state that calls an animation using your "ahegao" sprites instead.
Now I don't know if that is something M.U.G.E.N. can actually do, but it might be worth a shot. I know there is at least one version of Q-Bee that has a special thing with Minotaur where, if you linger on stage 3 of the standard doggystyle animation long enough, the horse guy walks in from offscreen and plugs her mouth. I haven't examined the code behind it, but I do know the horse guy in this case is actually in Q-Bee's own .sff and .air files, presumably coded as an explod.
Or you could just modify those other rapers to add extra code for ahegao states I suppose, but that wasn't really what you asked about.