What happens if I add DemandCenter campaign audience lists which reference prospects that are already participants in the campaign?

Nothing -- if you pause a 'Started' Campaign and add new lists to the Audience option to the Start Action, and if those lists reference prospects who are already participants of the campaign, those prospects will remain at the exact point in the campaign that they were in when it was paused. When the campaign is resumed, they will proceed through the flow(s) as designed. They will not restart flows, repeat Campaign Actions or in any way deviate from the designed behavior of the campaign.