The Night Stalker and Night Stalker Pro uses a standard 3/8" tripod screw. This is how the included ballhead connects to the tripod.
I got confused really quick as the RRS Anvil has tons of different configurations with different adapters, etc, so I don't know. If you find/use any ballhead that has a 3/8 hole at the bottom, you are good to go. Keep in mind the Night Stalker Pro is a very large/sturdy bipod, so I would typically recommend ballheads that are probably 40mm ballheads at the minimum. Also, the weight ratings of the Pro model are based on using the standard head, so make sure any ballhead you are replacing it with can handle whatever weight load you are placing on it.
The top of the standard Night Stalker Ballhead has a 1/4" connection. This is where the Arca Swiss plate attachment resides. In theory if a person is just looking for a different plate at the top, it could mount using the 1/4 connection as well but I believe you are looking to replace the entire ballhead.