I just wanted to ask if we can access an external MySQL server from Google App Engine...
The only way to communicate with other hosts is by using UrlFetch that only provides HTTP and HTTPS requests.
So, you can't do it out of the box.
Anyway, if you really need to access an external MySQL server database, you should consider to expose it through a Web API (RESTful, Soap web-services for example). In this way your data would be available also via UrlFetch.