Yes, it will fill the attribute for existing objects. This happens during the database synchronisation at start up.
This will include the SQL (example from a testapp using PostgreSQL database, where I've added an autonumber attribute autonumber to the entity myfirstmodule$entity):
ALTER TABLE "myfirstmodule$entity" ADD "autonumber" int8 NULL;
UPDATE "myfirstmodule$entity"
SET "autonumber" = nextval('myfirstmodule$entity_autonumber_mxseq');