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. :)
It's fine if it doesn't have any product code in it. You just gotta note down where it came from/what it came with and a link to the official japanese site that mentions the drama cd. Don't forget to put in the origin, company, voice actors, date of release, no. of tracks, etc.
Database questions are best asked in the Database Managers club club #828