Hi - Welcome to Flyertalk.

If buying an extra seat then you can do this once you already have a seat or it can be booked at the same time (you would need to do this on the telephone).

I would recommend that you don't bring the dress on the hanger - it should be put into a box to avoid damage. Not all aircraft types have places to hang clothes, or there may not be enough room for a wedding dress which tend to be larger than most hanging items.