How Can I Catalog Backup Sets in a Different 9i Database

No, you can't, because 9i does not support these "Re-catalog" commands below that are supported by 10g onwards:

CATALOG START WITH
CATALOG BACKUPPIECE
CATALOG RECOVERY AREA

You can't even use CROSSCHECK to make it become AVAILABLE, because the new database does not have the backup record of that backup set. But 9i do have the following commands:

CATALOG CONTROLFILECOPY
CATALOG DATAFILECOPY
CATALOG ARCHIVELOG

If you don't mind to use DATAFILECOPY, then you have a chance for your new database.

Putting the backup set under the directory of backup destination can't make RMAN to recognize it (tested), either. But there're still exceptions.

1. Without recovery catalog:
You can use the control file of source database for the the auxiliary database. Then
RMAN> crosscheck backup;
Most of the backup will be EXPIRED, but your backup set will be AVAILABLE.
2. With recovery catalog:
If the new database have its own control file, you can make it fake the source database by this:
RMAN> set dbid <source dbid>
And connect to the catalog to ask for the backup records of source database.
RMAN> connect target / catalog cat_owner/cat_password@catdb
RMAN> crosscheck backup;