Disney will not allow them to break up a contract- resales must be sold intact- Unless the resale is multiple contracts of smaller point value- then each contract can be sold separately. Usually they are listed as such as many people are interested in smaller contracts
If you want an add-on, it's really best to go through Disney. The closing costs involved in the resale of a small contract negate any savings you might get.
Another option would be to buy a larger contract and plan on renting out the extra points every year. Of course, those "extra" points will call to you .
Contracts can't be split. You are not really buying points, you are buying a real estate interest. The amount of points you have just represents the size of the real estate interest you have bought.
You can sometimes find a listing for a small amount of points or for someone selling several contracts that total 300 points.
While it's true that contracts can't be split if bought as a block, many for sale are actually a combination of a base contract plus add ons. It never hurts to ask.