Thanks for clarifying.
I'd keep those in the main Sony PlayStation CD, but make sure they're categorized (in the Category dropdown of the New Disc form) as Applications or Addon or add a new category for these kind of discs (but in this dropdown).
Alternatively, if there must be a new disc category, I'd choose a more generic name, based on the info I provided above (maybe "PlayStation Enhancement CD (Audio)" (as some of these discs contained not only upgrades but more cheat codes and maybe some other goodie).
Also, keep in mind these Upgrade discs existed for other cartridges, such as an MP3 player that also connected to the parallel port of the PS1 and was made by a chinese brand, and I'm probably missing some other HW that also used these Upgrade discs.
Anway, just my POV on the subject; hope it helps.