Look up the sources of the application you want to modify, and check for the po-file corresponding with the mo-file.
For example, if you want to modify konqueror.mo, you can find the corresponding file konqueror.po in the source package kde-i18n-language
Open konqueror.po in kbabel, (or an UTF-8 compliant editor) and change the strings in question.
Save the file and use the following command to compile:
msgfmt -o konqueror.mo konqueror.po
And then replace the old mo-file with the new one.
(you need gettext installed to compile the po-file)
For more information about this, please check out http://i18n.kde.org
Rinse