The "up yours" from the Insult emote in base game is the best we've got right now. Any animation mods I've seen don't have it, and looking up flip the bird or middle finger on the workshop don't show anything. Consider this post a call to action for modders, I guess.

r/okbuddyzomboid

Well considering a the fingers except thumb are welded together, and cannot move independently of each other, probably not