There's a couple of ways to do that but ultimately it's manipulating bones in KKPE advanced mode. To make cheeks clap, you can just open KKPE advanced mode on your character, go to bones, and search "siri". This will show you all buttcheek bones. Select one of the bones and tick "symmetrical", then click between the position, rotation, and scale tabs and tweak some of the values in each to see what they do.
To animate this, you can open timeline and scroll through the left side list until you see a section for KKPE. Should see interopables for things like Bone Scale, Bone Position, Bone Rotation, etc. Click on the bone you wish to animate in KKPE, then shift+click on the timeline rows for Bone Position, Bone Rotation, and Bone Scale.
This will add active keyframes for each component of that specific bone into Timeline. To animate this, all you'd have to do is scrub some amount of time ahead, tweak the cheeks to your desired characteristics in KKPE bone editor, then shift+click the rows once more in Timeline to create new keyframes with the new position/rotation/scale data.
I may do a video showing how I rebuild a scene to fit with different characters and outfits. The tricky part with doing this is I'd have to do an awful lot of editing so you can clearly see what I'm clicking on, what keyboard shortcuts I'm using, etc. Another issue is many times it's just trial-and-error in which I am attempting to find the best bone to tweak for a specific desired look, so it's a lot of back and forth and undoing and redoing, lol. I'm currently rebuilding one from ZeroKK and nearly got it finished, but I'll try another and document what I'm doing.
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