Sure, that will work. You’ll need to create an aggregate device that combines your HDX system and the H9000 USB Audio. Then you will have access to the USB I/O on the H9000 from Pro Tools: https://cdn.eventideaudio.com/manuals/h9000/2.2.11/content/setup/usbsetup.html#using-the-h9000-with-your-existing-computer-audio-interface