i am not a plugin or theme developer, but i do develop php.
an html form that sends post data to your plugin's settings file would achieve this temporarily, but actually writing permanent settings would be troublesome.
the best way imo would be to have your plugin create another column in the wp_options table in your database. or create another table if you feel that is better.
have your plugin/theme check it for settings when it starts, have the options page insert settings when a form is submitted.
you obviously would want to keep database calls to a minimum, and use wordpress functions over mysql functions where possible.
to create the menu link in the dashbboard, you would use this.