1. You right click your NPC and choose "Save as". It will be saved under Exported Creatures in the Creatures & Items panel.
2. If you right click your NPC and choose "Follow party". It will always stand beside the characters when you load a new scene. But, as said above, it will not follow inside the scene, so you have to manually move the NPC when the party moves, or possess him and walk him yourself beside the party.
3. I have for some time tried to get Larian or a modder to make it possible to make NPCs actually follow behind a character inside the scene as well. But nothing so far, unfortunately. A modder had successfully made this possible by adding a status effect to a NPC, which made him follow. But he quit and no one have picked up that ball, unfortunately.