I think you need the commas as you stated. However the whole sentence seems awkward to me. The "in addition to or rather than" seem to cancel each other out. It seems like you are giving instructions that all conference-attending company personnel whether they are staffing or not will register and pay the fee.
This seems clearer to me:
If company personnel will be attending these conferences, even if they are staffing the exhibit, they must register as attendees for the event and pay the appropriate conference registration fee.
Or:
Even if they are staffing the exhibit, company personnel who will be attending these conferences must register as attendees for the event and pay the appropriate conference registration fee.