Once you are a member, adding on is really easy. You call, tell them what you want, give them your deposit and the send out the paperwork.
There are no closing costs for add ons and the points are in your account within a very short time (mine were there within hours) to make a reservation.
They still have to send you paperwork that is the same as an original purchase. So, if you want to get it started and then just take your time with the papers in hopes that new incentives will come out you can. I did that in June with my 50 point add on.
Matter of fact, because I was using my Disney Visa for the purchase, I asked that my final payment be delayed a month so that it would miss the statement closing date. I was getting 6 months, 0% interest, and by holding off that one month, it really gave me 7 months.
Of course, after I signed and the new incentives came out, they were not as good so I kept what I had.
But, in March when I was purchasing, the incentives did get better and I had the contracts re-done.