I had all my ADRs then added the Dining Plan later.
When I called dining to make my ADRs, the dining lady asked me if I was on the Dining Plan, I told her not at the moment but we may be adding it and would that cause a problem. She said not at all.
When I called travel to add the Dining Plan, I asked if my previous ADRs will be effected. She said not at all.
I did not have any Dinner Shows booked, so I don't know how that will work for you.