Is there an official way to add extra fields on a content type on Aegir.
To be more accurate, I want to a comment (text) field for the 'sites'.
It's Drupal, so yes you can add an extra field to the site content type on
Warning: you must enable first the
field_ui module, as it's disabled by default in Aegir installation profile.
I started a documentation page about this: http://docs.aegirproject.org/en/3.x/extend/frontend/