[neudof]

Not sure if you can change the default, but try this for adding other options:
Go to the url bar and type the web address of the search engine
When the page has loaded fully, do a long tap (hold) on top of the search field, and the context menu will have an "add search engine" field.
Now, at the main page of opera, you can use the down-arrow next to the search engine field to switch between search engines.