Admin Control Panel: Add/modify fields?

Just wondering if there is an extension that would allow me to add fields to this area of the admin CP:

CP Home › Admin › Weblog Administration › Weblog Management › Edit Weblog › General Weblog Preferences

Specifically, I would like have a text area that allows for a longer “Weblog Description”.

Alternatively, could you suggest a good tutorial(s) and/or extension(s) that I could disect in order to help me learn how to write my own admin CP extension?

Thanks! Cheers, Micky