@Seck, once you enable NPC sheets for the campaign and create your NPC sheet, you will be able to see a button at the bottom when you're posting. By clicking on that button, it will open up the NPC sheet and you can then click on which ever NPC your want to post as. Their image will appear in the text field and then you can post as them. You can then, repeat the process and add other NPCs into the same post without having to post for each one.
You can keep notes and details about each NPC on your NPC sheet and look there anytime you open to make a post for an NPC without switching back and forth to another thread.