Hello,
This is not officially possible.
Potential work around include:
- Creating new user on LDAP and then use transfer-ownership to move the files to these new users, if the only user data you have is files.
- Using user_migration application to export the local users, create new LDAP users, import into them. Might be slow depending on the amount of data, will not work for every kind of data, only what the application currently supports.
- Tinkering with the database to tie the local accounts to the LDAP ones, but this is uncharted territory and needs deep understanding of Nextcloud database tables.