Hi Victor Hugo (really?:))
Thank you for your message.
Just to check if I understand your question -> Lets say you have a field in your document type as Textbox form control and you would like to add some custom additional settings/information to this field?
If you you could add it easily through Site manager -> Development -> Form controls -> Text box -> Properties tab. And you will see list of all settings of this particular form control and you are free to add one.
Also it could be better if you would develop your own form control instead of changing ours. You can based your form control on ours. Please see following guide for more information regarding this ->