This is a modified tutorial from Dromed Central. Author: John_The_Taffer

Go into the Object Hierarchy. At the top of the dialog box, in the field next to “Show Tree” find MetaProperties. Click Add. Choose “Sitting” as
New Object Name and click Ok. Click “Sitting” and “Edit” to the right.

Add→Al→Ability Settings→Idling should Fidget. This box should be unchecked.

Add→Al Core→Standing Motion Tags: don’t write anything.

Add→Creature→Current pose:
Type: Tags
Motion: Stand2Sit
Frac:1.00
Model scale: 1.00
Ballistic: Unchecked

Create an Al and go to it´s Properties. Add→Metaproperty→Sitting. Click Done.

Your Al Will Sit!!

NOTE: It is important that you save the gamesys(preferably with a new name) if you want to keep the changes in the Metaproperties-hierarchy. Then use the command set gamesys followed by the name of your custom gamesys to make the mission use this new one.