I have two cats who have chosen to switch, in their own good time, from an indoor litter tray to specially dug outdoor places, which I have filled with John Innes compost - I think you have to offer something as easy to dig in as cat litter
my third and most recent cat is weeing in the tray but pooing outdoors, and I expect he will switch to outdoors completely eventually
but I do think this change should not be rushed - and to be honest when its very wet and muddy I wish all three would use the litter tray, as I get muddy pawmarks everywhere
I use Oko Best Clumping, as although it tracks everywhere, there are no nasty whiffs at all, it is lightweight, and lasts ages and ages