Drama CDs fall under media and media items must have a JAN/Catalog code on their database entries to be added (Doujinshi are the exception though.). It seems like your drama CD could be bundled with a game or something so it can't be split from the main media (a game etc. that has their own JAN/catalog code) either.
Just to add on, if you need help for identifying origins of items, you can ask in club #99 too. :)