The way that BB's crew can "extract" the devil fruits from people is by eating them. If you think about it, then Big mom ate mother Caramel and got her fruit, the soru soru no mi. so thats probably how you get it from dead people. because the fruit might still be manifested in the person until any amount of time past their death, possibly even until it goes back into the wild.
What do you think?? idk if this was common knowledge but