Hi @OlympicMtnBoy! The packrafts are designed with a fairly narrow cockpit to minimize weight and improve paddle clearance, so a pack will probably not fit between your legs. If you build a Large size, however, you will have an extra 20 cm (8″) of length and a day-pack would probably fit in front of your feet or behind your back.
An alternative would be to add an airtight zipper so you can store most of your gear inside the tubes and just have the essentials out where they can get wet.