Check the AnimGraph of a Metahuman and you’ll notice that there is a Pose asset called mh_arkit_mapping_pose created based on the mh_arkit_mapping_anim.
What you need to do is pretty much the same thing, where in your DCC you create a setup of facial expressions for each frame ( using a facial rig created inside your DCC ), then you can import the results in Unreal.
For convenience I strongly suggest you to use the same naming template from the Pose Asset, so that you already have the template properly mapped and the facial board controls will work as expected.