If such a site has a search field, I use the keyworded bookmark feature from FF: just right-click in the field and select "Add a keyword for this search", give it a name and a keyword, preferably a short one.
Since the awesome bar also searches your bookmarks you can now directly open search results: just type in the keyword, a space, and the search term as you would enter them in the input field, and finally hit enter.
Of course this does not add anything to list of search engines... however, I never used that extra input field anyway, since I am much quicker typing "keyword search terms<enter>" than "ctrl+k search terms, click to select engine, <enter>". The bookmark-approach works very well with the almighty vimperator
However, some sites such as denic.de have built their search form thus you need javascript to submit it. I find this very bad behavior, and it breaks the keyword search...