It is possible to add additional search engines to the applet if you have root access to your Nokia 770. The configuration files for the search applet are located under /usr/local/mis.

For example, for Google it looks like this:

--------------
<search name="Google"
icon="/usr/share/icons/hicolor/scalable/hildon/qgn_indi_search_google.png"
action="http://www.google.com/search"
user-input="q">
<input name="btnG" value="Search"/>
<input name="hl" value="en"/>
<input name="safe" value="medium"/>
<input name="client" value="ms-nokia-wifi"/>
</search>
--------------

The coloured parts refer to the search URL as follows:
http://www.google.com/search?q=<search-string>&btnG=Search&hl=en&safe=medium&client=ms-nokia-wifi

In analogy to this format I made up a configuration file for Wikipedia:

--------------
<search name="Wikipedia"
icon="/usr/share/icons/hicolor/scalable/hildon/qgn_indi_search_wikipedia.png"
action="http://en.wikipedia.org/w/wiki.phtml"
user-input="search">
<input name="go" value="Go"/>
</search>
--------------

Save this file as wikipedia-search.xml under /usr/local/mis and reboot your device. And you're done.

BTW: The Wikipedia icon was already there on my Nokia 770. Seemingly, the developers at Nokia were close to adding the Wikipedia search engine.