How to add additional custom translation for a payment processor?
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 with this command: msgcat --use-first *.po | msgfmt -o civicrm.mo -
replace the original civicrm.mo file on the server with the newly created one
clear cache


This is working for me. The extension is translated.