This function is probably the easiest way: UnitAddItemToSlotById
It creates a new item in the slot specified.
I don't know if you can do that in GUI, though. Perhaps this workaround may help:
Before removing the normal blink dagger, create 6 dummy items and give them to the unit. Remove the blink dagger, add the disabled blink dagger, then remove all 6 dummy items. Extra dummy items will appear on the ground, and the unit's entire inventory will be full, so the disabled blink dagger will appear in the correct slot. Then you remove the dummy items so they don't sit around everywhere forever.
I'd rather use a custom script or JASS though.