Declare the behavior of your new rooms in the tiles.js file, each tile type needs to be declared in this file, the names must match up with names used in the map data. Look at the existing rooms to get an idea of how to customize them. The most basic things are description and look properties. Use these communicate with the player about their surroundings. You can also add items or enemy declarations to the tile objects. Items must get declared in items.js file.
For a little more intermediate practice you could do some tinkering with commands inside of commands.js. With some creative use of commands you could even make entirely different genre's of text game venturing out beyond the RPG / Adventure / Map exploration of the example game. You could make a market simulation, or some kind of primitive computer terminal based game, or some other kind of puzzle game. The possibilities are only limited by your imagination.