NPCs are defined by a creature template, a UTC. Their position/orientation is defined in the module's GIT file. Basic idle animations are controlled via the scripts defined in the UTC. For background NPCs that are just static/standing around, you should be able to duplicate an existing NPC in the module and edit their specific details like tag/resref, name, appearance, etc., then add the positional detail for the new creature in the GIT. If you want more advanced behaviour, like patrolling/wandering around, this is done via scripts and waypoints. Again, looking at how existing NPCs work will get you most of the way there. Once you have everything in place, you will need to package up a copy of the entire module with your additions as a MOD.