Internal/External project table settings are handled in V4. Once the file is exported, it's a done deal.
To be able to set the project table outside of V4, where it was created, is to contradict (one of) the purpose(s) of having the project tables in the first place.
Have your customer export the file with the corect settings, or ask them to send you the project tables. Oh, and by the way - they'll probably do neither.
