You can't buy extra points from Disney to use for a year as you suggest. What most would do for something as you describe would be one of a couple things.
If points owned are adequate, many would to use the banking and borrowing rules to get up to three times the normal annual amount and use them that way for higher point reservations. Of course, that can pretty much deplete's the points from the other two years.
Others will have another member transfer points to them for such occasions, for a fee though technically there is supposed to be no monetary compensation for transfers. You would post your needs on the rent/trade board and pay someone for the extra points for that year.