So I am looking at PC Sims Hairbrush and I was wondering what I would need to change in order to make sims to sit down before they complete the animation.

Sort of how they should look for a seat before they do decide to stand up

Quite a lot - animations are hard. Presumably Kate did the animation with the sim standing.

You also can't extract animations for editing, so you'd have to make the animation from scratch.

You know, since it's slightly related, I've been vaguely curious about cross-importing animations (despite knowing nothing about creating, especially Sims animations). I know that it's incredibly hard/impossible to import animations from TS3 to TS2 (why importing from TS4 is impossible I understand, the fact that they have selectable height differences alone rules importing out), but I was wondering what aspect of it ruled it out (or the largest, anyway, since it's probably numerous things).

I get that transferring/importing things like traits is impossible because the coding for the two game's programing is so vastly different, and that the two end result models of the sims are vastly different looking, but what about just the basic "skeletons" for the movement? Like, just the bare bones ones you see in behind-the-scenes stuff when animators are creating either a stick-figure or wire-construct looking model to get the basic movements down before fleshing it out.

Or is the fact that TS3 has different body types (extremely thin, bodybuilder muscular, extremely obese, etc.) what rules it out (since you would need a different animation for each body type, like how you need seperate ones for different heights, ie, why some animated CC objects in TS2 go a little wonky when kids use them)?

Well shipping animations from 2 to 3 must be possible with how much EA did it.

No idea if anyone outside of EA has done that however.

Well - EA have the source code. We don't.

EA has not transferred animations from TS2 to TS3/4, from what I can see. The animations are similar, but yet different enough to have been made from scratch. The skeletons and rigs are very different from TS2 to TS3/4. They may have transferred some animations from TS3 to TS4, since the skeletons are more similar, but most likely the animations were made from scratch there as well.

It is possible to tell an the object to run a set of ingame animations before they start and after finishing a custom animation, such as walking up to the object, picking it up, do the custom animation, and put the object down. Most object creators use ingame animations for their project, combining them in new and interesting ways. Only a few actually make custom animations (they're very difficult to get right in TS2, because of the skeleton mechanics).