First we have to load the terms on OpenERP.

Log on OpenERP and ensure you are in Technical Features group

Synchronize translations in

Settings -> Translations -> Application Terms -> Synchronize Terms

Then use the following command line to generate your po file:

server/openerp-server --log-level=error --i18n-export=/tmp/<lang>.po -l <lang_code> -d <database> --modules=<module_name>

Ex.
lang_code = fr_FR
lang = fr

Then you can use msgmerge to merge the new po file with an existing one.