This is something i been waiting for a long time for them to implement but never seem to happen.
I really wish they made a small button on the inventory screen or a subcommand so when you have a retainers inventory and you do that it will check your entire inventory, and if the retainer has also have that item it will move all of them into the retainer stacks.
Would save so much time compared to having to manually check each individual item as it is now.