Do you have a translation file of your starting language? Suppose your untranslated application is in English. Copy the .pot file (e.g. "project.pot") to have extension .en.po ("project.en.po"). Then open this file in POEdit, click in every line and press CTRL+B to transfer to non-translated English text to the column with the translated strings.
I guess it should be possible to use the .en.po file without the CTRL+B trick, but I never found out how...