Doomspeaker already quoted the appropriate bit, but I'll add some more detail.
Yes, you can make your own rooms and levels for the game. We actually use a very simple, free, open source tool called Tiled. When building rooms, you have the ability to place static (non-random) objects and/or objects that are affected by the randomness system. So, you could completely handcraft a room or you could make a room that is completely randomized based on the content and parameters you put in.
You can also tile the floor and have multiple variations that the random system can choose from. The walls are procedurally generated, since they are a mix of 2D and 3D pieces.
"Levels" really just consist of data that references all given rooms a level can contain, some configuration parameters, whether or not any rooms are required to be included, etc.
All of this will be documented at some point and made available with the other general modding information.