As documented here you can download your data into a blob:

appcfg.py download_data --application=[app-id] --filename=[output dump file] --url=http://[app-id].appspot.com/_ah/remote_api  

And then upload to another app:

appcfg.py upload_data --application=[app-id] --kind=[kind] --filename=[input dump file]  

Or to work with structured data first generate the bulkloader file:

appcfg.py create_bulkloader_config --filename=bulkloader.yaml --url=http://[app-id].appspot.com/_ah/remote_api  

and make sure to set the connectors to CSV

Then download the data:

appcfg.py download_data--application=[app-id] --config_file=bulkloader.yaml --filename=[csv file] --db_filename=progress.sql --kind=[which entity]  

--db_filename is important for large uploads to avoid re-uploading the same data