I inherited a lot of (900) 6mm Creedmoor virgin brass but I only have a 6.5 Creedmoor rifle. Is it possible to just open the throat to accept 6.5mm bullets?
It’s called the case neck. The throat is inside your neck. Yes, it’s ok to do that, just use plenty of lube and expect increased seating pressure afterwards for the first firing cycle.
Probably better off using a mandrel and not the expander ball in a sizing die (assuming he is using an expander ball, which many recommend against if using bushing dies).