Adding custom settings per blog

How do I go about adding a custom setting in the settings section for a particular blog / site i.e. going to a the admin page like “/wp-admin/network/site-settings.php?id=6” there are many settings listed. I want to add my own custom setting (just some text) for a particular site. I assume I can do this by manually adding an entry to a database table. Which database table must I add this entry to and once added what php code can I use to access this setting?

Thank you.

Andy