Is it possible to set up an NPC in a game, but keep them hidden from the players until it is time for them to start posting?

I have ideas (including portraits and background) for several NPCs in a game I run and I want to set up the details now (rather than have them on a scratch pad or private thread) but keep the details concealed from the players, is this possible?

As soon as any character is added to a game, they appear in the cast list.

However, see this thread, for a feature that might be of help in future: link to a message in another game

Excellent, thanks for the re-direction...