Hi Ed,
I agree - it's something that really ought to be supported. You could suggest it in the Idea Place.
The Goods Issue posting transfers to GTS automatically in the background. At some point (usually overnight), you need to process the Domestic Issues (find that one in the "Background Processing" tab of the "System Monitoring" area). That a program tries first to find duty-paid stock to satisfy the Goods Issues. If it can, it reduces the DP stock quantity; if not, it tries to find duty-unpaid stock. If it finds DUP stock, it creates an entry in the work-list "Domestic Goods Issues from Customs Warehouse", in the "Customs Warehouse" tab of the "Special Customs Procedures" area.
From the work-list, you create one or more Import Declarations to reduce the DUP stock and declare to the Customs Authority for duty payment.
Hope that helps.
Regards,
Dave