While creating a pitch, you can add new files, songs, and folders to it. The easiest way to do this is to drag and drop whatever asset you'd like to add to the pitch.
There are also Add to pitch options located in the header of playlist folders and song projects if you are currently working on a pitch and have the pitch edit modal open.
You can also access the Add to pitch option by right-clicking on a song or file.
You can also access the Add to pitch option from the more options menu on selected songs and files.