Bakery doesn't pre-sync accounts I want to use the Bakery module to login and synchronize users, but I am not able to synchronize existing accounts. Users that exist only on slave cannot login confirms Bakery doesn't provide pre-sync facility.

May someone help me to synchronize existing user accounts, possibly at database level, for example with drush rsync or sql sync?
I would go with the migrate module, it's no syncing but export and import in two steps. But i cannot tell you if existing users will be replaced, skipped or doubled ..

Maybe account_sync does what you want.

Also have a look here How to sync users profiles and user tables in 2 drupal sites.