Can I add music from an old itunes account to my new itunes account?

Downloaded music can be integrated into another music library by importing the files directly. They aren;t tied in any way to the account.

click here ➜ Import music from a computer to Music on Mac - Apple Support

If you want to merge the actual purchases and history from the iTunes Store rather than the files, then you will need full access to both accounts and then follow the steps in the link below.

click here ➜ Migrate purchases from one Apple Account to another Apple Account - Apple Support