Apple - How can I add a new search engine to Safari without any 3rd party add-ons?

No, the only options are Google, Bing, and Yahoo.

If it is of any help, DuckDuckGo have their own extension on the Apple website for Safari extensions.

The Safari Extension anysearch allows you to change the search engine used by the Safari search bar/unibar, without any hack-ish stuff such as changing the hosts file.

MacHints just did a post on this. The basic premise is that you redirect Yahoo! Search via the hosts file to the search engine of your choice. It works like a charm.