Yep, you can add search engines by adding a web-search-whatever.desktop file into /usr/share/applications/ if you want you can base it off of one of the pre-existing web-search files

You can add your own (small medium and large) icons by saving them into /share/themes/blanco/meegotouch/icons/

I've personally made a Duck Duck Go search plugin (including icons) which works nicely. I've been meaning to package it up as a .deb to release to the public. So if anyone is interested in that I'll get more motivated and do it.

EDIT: Duck Duck Go .deb available here