You can have more than one graveyard,I have used graveyard lots saved from other worlds in X world.I made several of my own graveyards with a few having dead sims from various worlds placed into one cemetery.You can go on a favorite dead sim collecting spree befriending ghosts till you get the "Pick up tombstone" interaction after which save the sim to library it will keep inventory intact exit to main menu place him in a new town grab your fave dead by repeating those steps till you have a bunch of them from various worlds.Afterwards place that sim on a empty lot or in case you do not know building even one wall piece in edit town is enough to count as a house to place a sim.Anyway you build a custom cemetery placing the tombs from his or her inventory where you like then after you are done you can change it from a residential to a community lot/graveyard.
Can also do the same thing if you want your own created ghosts even a cemetery of hottie ghosts.Make a bunch of pretty sims kill them in various ways have a living sim collect the tombstones keep repeating till you have amount you want and make your custom graveyard.Beautiful Banshee Resting Place :p
Graves placed indoors become urns,graves outside stay tombstones