Writing at Developing for Dynamics GP, Viswanathan Neelakantan shows How to modify a field value in a line when generating the EFT file?
How to modify a field value in a line when generating the EFT file? – Developing for Dynamics GP – Site Home – MSDN Blogs
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Archived post from 2012. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.