There might be a misunderstanding: the offline translating work doesn't let you import messages from another wiki. You need to use a PO editor, so if you translate first on your local wiki you might as well just copy and paste your translations directly into Special:Translate here.
Just to make sure, are you aware of Special:SearchTranslations? That can help you find a string that needs to be translated. So you could do your local testing with the translations which are currently in MediaWiki code, identify the missing translations in their context and then find them here, input your translation into Special:Translate.