You can extend the schema by using a schema file that contains customized definitions. In general, the best practice is to modify the existing 99user.ldif file in the install-dir/config/schema directory to add new definitions. When you update schema elements using LDAP, the new definitions are written to the 99user.ldif file.
Alternatively, you can create a custom schema file and save it to the install-dir/config/schema directory. The directory server loads schema files in alphanumeric order with numbers loaded first. As such, you should name custom schema files as follows: 00-99filename.ldif. The number should be higher than any standard schema file that has already been defined. If you name custom schema files with a number that is lower than the standard schema files, the server might encounter errors when loading the schema.