Hi William,
Typically, in order for a pre-existing medical condition to be covered by Walt Disney World trip insurance, it must be purchased within 14 days of your initial trip deposit. You do have up until final payment on your trip to add insurance. For more information, you can view the
FAQ: Trip Insurance on the Walt Disney World site.
However, if you have any questions I would highly suggest calling Disney at 407-W-DISNEY or contacting your travel agent. They'll have the specifics of your vacation and can help you determine what the best way to proceed is.
If you determine that Disney Trip Insurance will not cover your needs, there are many independent travel insurance companies that can be found online. However, be sure to read the fine print! They all offer different levels and types of coverage!
Hope you have a great trip!