Unfortunately, those are the only ones that come default with the iPad. Sorry.
The only suggestion I can make is to make a bookmark for the search engine that you wish to use. That way, you'll always have it handy to open versus having to type it out. You could even save that bookmark to your Homescreen so that, when pressed, it will open Safari right at that page.
Best solution I can think of... Hope this helps.
Marilyn