Just after starting the bounty i have found what i was looking for:
To merge two .po files you'll need to use some shell magic. I did the following:
extract the original civicrm.mo file to a .po file with this gettext-command: msgunfmt civicrm.mo -o civicrm.po to an empty folder
paste the .po file of the extension to the same folder
merge the two .po files ...