Do the additional participants in the Large Meeting Room and Webinar add-on stack from the initial maximum number of participants of the base plan?

No, they don’t. If a client has a Pro License and avails of a Large Meeting Room 500, the maximum number of participants in 1 meeting room is still 500. If a client has a Pro License and avails of a Webinar 100, the client will choose the role of the 100 participants allowed if they will be panelists or view only participants.