Edits in *.po/*.mo in default language

Hi,

I’m using the WPML blog version + po/mo files for theme localization. My theme already includes a default.po + mo with the english strings of the theme, but I would like to change some of the originals english strings. It would be great if I could do that in the po/mo files instead editing the template files. The problem is, I cannot change texts in the default.po with Poedit (only translate them but the translation will not be displayed in the english version of the theme). Creating a new en_US.po/mo didn’t work either, it seems not to be recognized by the theme, which still uses the default files for the english version.

Can anyone give advise what is the best way to change the default strings?
Any help is appreachiated 🙂

Thanks,
Panda