Hi Reg,
Adding a track to an existing MIDI is done using the MIDI 'Multi Recording' function which is explained starting on page 69 of the downloadable 'Genos Reference Manual' (this is not the 'Genos Owner's Manual' that you received with the keyboard). If you haven't already got this, you can download it from the 'Manuals' section of the page at this link...
https://uk.yamaha.com/en/products/musical_instruments/keyboards/arranger_workstations/genos/downloads.html#product-tabsThe process requires you to open the Genos equivalent of 'Song Creator' which you do by pressing MENU > MENU 2 > touch Song Recording > MIDI [Multi Recording].
When the Multi Recording display opens it contains a blank song (New Song), and to use an existing MIDI, you must now load it into the recorder, as explained in instruction #2 on page 69. This applies even if you have already loaded your song in the Song Player! This is different to previous Yamaha arrangers if you are used to these.
After you have loaded your song you can record individual tracks as required using the rest of the instructions.
Post back if you have any specific problems.
Regards
Ian