I assume that effect would have to be generated from mirrors. If my memory of the HM blueprints are correct, the organist and staircase scenes directly behind the Hitchhiking Ghosts animatronics, so there probably isn't much room behind the existing wall to expand that scene.
During the old WDW staircase scene, the Ghost Host delivered the lines "We have 999 happy haunts here, but there's room for 1000. Any volunteers? Final arrangements can be made at the end of the tour." (or something like that)
It's somewhat similar to the dialogue used at DL where guests are walking past the changing portraits before boarding the Doombuggies: "Several prominent ghosts have retired here, from creepy old crypts all over the world. Actually, we have 999 happy haunts here. But there's room for 1000. Any volunteers? If you insist on lagging behind, you may not need to volunteer."
With the rumored installation of DL's changing portraits in the former Sinister 11 gallery, I guess its possible that the old stairway narration could be moved to the new portrait hall. They could also eliminate the original WDW "999 happy haunts/final arrangements" narration and replace it with an edited version of the DL "999/creepy old crypts" spiel.