Hey!
So as far as I know, vanilla Bitsy doesn't contain a function to store rooms' IDs as variables. However, you can do some roundabouts to achieve kind of similar results. My way of doing it was through a bunch of branching lists. For example, in this game, the player is sent to a random room each time they go through an exit, but some conditions determine what room they will appear in.
So, as I see it, you can use a branching list as a 'variable' of sorts, that will contain all your rooms, with different conditions attached to each of them. Then when the exit is triggered, the game will run through all of the conditions and determine which room to send a player into. This is super cumbersome, though, and may even make Bitsy lag and crash, so based on my personal experience, I would say do it at your own risk.