Betrayal at House on the Hill is kind of notorious for the rules not quite covering every situation clearly.
I think a strict reading of the rules says that you can fall in, and it gets marked as "beneath" the Collapsed Room so you can fall in later too, simply because there are no special rules for this situation.
That said, there are several non-game-breaking options for if/how you can fall into the elevator, and I think you can honestly just pick what you think is the most fun:
Allow falling into it, and place it somewhere it can actually connect. (Pretend you fell in, then the elevator activated and moved.)
Allow falling into it, and if there's not already a basement, find an actual basement tile to connect it to.
Allow falling into it, but if it can't connect, the player has to actually activate and move the elevator to get out.
Ignore all this and find a "real" basement room.
And then you have options for what happens after the elevator moves, and someone falls through the Collapsed Room again:
They still somehow end up in the elevator, even if it means falling up.
They draw a new basement tile, and that one is under the Collapsed Room now. (This is consistent with how the elevator works with doors: if you go through a door and the elevator is there, then it moves away, then someone goes through that same door later, they find a new room to put there, they don't magically end up in the elevator.)
The two first options are the strict rule reading mentioned above.
But it could also be more fun and flavorful to pick a different option, like 2 or 3 for the first choice (either connect to a new basement tile, or have to move the elevator) and 2 for the second (find a new room to put beneath once the elevator moves).
Given the frequent vagueness of the rules, and the fact that none of this is really going to break the game, I'd say go with whatever suits you.