You should open your Nabble application and look at the HTML source. Then search for "search-box" and you will find a HTML form element. For example, the code below belongs to the Nabble Support forum:
hi hugo i was wondering if its possible for the current search engine to search or index other sites that we want?
that way i can get it to search my main site and forum
if its not possible or will take to long, then dont worry about it.