The following is a workaround.

1. Set DuckDuckGo.com as your default search engine.
2. Install TamperMonkey extension.
3. Enter the following script into TamperMonkey.

This will redirect your search from DuckDuckGo.com to Ecosia.org automatically. So Ecosia.com becomes your default search engine.

Note:
NEVER use scripts from unknown sources.
This is just a one line command, redirecting you.
I'm posting the full code here so everyone can review it.

Code:

// ==UserScript==
// @name         Ecosia Default Search Engine in Opera
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Sets Ecosia as default search engine by automatically redirecting you
//               when you search in DuckDuckGo.
// @author       Hagbard Hednig
// @match        http*://*duckduckgo.com/*?*q=*
// ==/UserScript==

(function() {
    location.href=('https://www.ecosia.org/search?'+window.location.href.substring(window.location.href.indexOf('q='),window.location.href.length));
})();