While you can make reservations before actually closing, I don't think you can actually purchase an add-on until you have closed but I'm sure you can get the process moving along and maybe even close both contracts simultaneously (I know that some have purchased 200 points divided into a 150 point contract and a 50 point contract).
If you have an interest in more points, just call your guide and let them know what you'd like to do. I know they'll accommodate you every way possible.