We are doing a 3 day/4 sea, and I added an extra night so we could fly in early...BUT, I had to book the extra night separately, because I did not wish to pay for park tickets for the arrival night.
This meant I had to book our airfare separately, as well. It actually became a better deal. Even with paying for an extra night, I saved over $200 by booking our airfare, and I control when the flights are.
We are booked on ME for the arrival, and still kept our transportation vouchers to get to Port Canaveral and for our return. The CM I did the one night booking with said he would make a note in our reservation that we had a land/sea starting the next day, and that we would like to stay in the same room.
I intend to call and verify that we have this request closer to time. I'm gathering we aren't the first people with a request like this, so I think it shouldn't be a problem.