Is it possible to export my Chromium search bookmarks to Opensearch format?
As far as I know, you can't export them.
However, you can pass them from one Chrome profile to another. The file is located in
%localappdata%\Google\Chrome\User Data\Default\Web Data. Just copy it from the older profile to the new one.
The file is actually a SQLite Database, so technically you could write a script that reads the file and exports it.