Can NPCs become force sensitive and if so Jedi or Sith?

No, they can't.

To add this question. If you NPC'd a Jedi or Sith, would they by chance be a FS NPC, or would the server just delete that portion?

As far as I know, the FS flag only exists for players and not for NPCs at all. However, even if it were to, Force abilities are not usable by an NPC, so it is a moot point.