Yes, you can use FaceFlex with any 3D model as long as it is properly rigged. The add-on requires a rigged face model to work, and you will need to pose the model according to the ARKit 52 blendshapes using the free reference guide provided by FaceFlex.

FaceFlex is an add-on for Blender that speeds up the process of creating Blend Shapes for ARKit & Wonder Studio.

No, it does not. You need to rig your face model and then pose and add keyframes on the timeline according to the ARKit or Wonder Studio Blend Shape poses (provided in the free reference guide). Then FaceFlex will convert and rename the keyframed poses as shapekeys or blendshapes.