Can you make your own NPC's?

You make your own NPCs by drawing their sprite, making them a combo with a transparent background, then put that combo on layer 1 or 2 onto your screen.

To speak to them, you'd make an identical screen where there's an invisible guy on the screen with a string. Surround each screen you're using with the necessary direct warps and such, and make sure your Dmap has a level number greater than 0, because if it's 0, the guy won't say anything, much less even appear.