This is a CLI wrapper for the WordPress Importer found at https://wordpress.org/extend/plugins/wordpress-importer/
It's reusing most of the code of the original import but allows it's execution on the command line.
It's required to have the WordPress Importer installed.
Installation:
Grab the wordpress-importer-cli.php and place it in the wordpress-importer plugin folder ( usually wp-content/plugins/wordpress-importer ).
Usage:
Call the script on the command line with the following arguments
Argument --blog: Blog ID of the blog you like to import to
Argument --file: Full Path to WXR import file
Argument --attachments: Import attachments (true/false)
Argument --user: Username/ID the import should run as
Argument --author_mapping: empty or php file with User mapping array $cli_user_map = array( <old_user> => <new_user_name/email/id> ); defined. if the file does not exist it will be created for you
The script also tries to find the best matching user on the current blog based on the similarity of the users. Please make sure to check the author mapping file before running the import. 
Sorry, not much documentation at this point and all needs a lot of testing. 
Feedback welcome!