Websecurify can be customised in many different ways depending on what
you want to do. The formal documentation is planned to appear around
version 1.0. However, in general, if you know how to write Firefox
extensions you already know how to customise Websecurify. A good
source for all that you need to know can be found at
http://developer.mozilla.org/.
Check
https://developer.mozilla.org/en/extensions for extension
development topics. I understand that it may seem difficult initially
but if you persist you will soon realise how simple in fact the whole
process is and how much power you get at your fingertips. Also on the
flip-side of things, once you know how to extend Websecurify you can
extend Firefox which is an awesome skill to have these days.
You can also check Websecurify's source as well. It is a good source
of information as all features are implemented as extensions.
I promise to release a formal extension development guide very soon.