You would need to write an
action plugin and most likely handle the
HTML_REGISTERFORM_OUTPUT event to add your field to the form and also the
ACTION_ACT_PREPROCESS event to process the extra field after submission. For inspiration take a look at the
captcha plugin, which does something similar in order to prevent bots manipulating a wiki.
Note. The form events have been altered in the development version of DW. I have mentioned the new event above, the previous style was to use "*_INJECTION" events. The injection events are in the current release. I think they are deprecated. DW is entering a new release cycle (development code has just been frozen). If you are to build something to do this, I'd strongly advise commencing your development against the development version and deploying after the next stable release (in a month or so). For more details on the development version, refer
development resources