$this->addOption( 'username-interwiki', 'Use interwiki usernames with this prefix', false, true );
I cannot find any usage of 'username-interwiki' other than in the constructor of BackupReader
I do however find username-prefix
if ( $this->hasOption( 'username-prefix' ) ) { $importer->setUsernamePrefix( $this->getOption( 'username-prefix' ), !$this->hasOption( 'no-local-users' ) ); }