from MSDN: "All the parts must use the partial keyword" (
Partial Class Definitions (C# Programming Guide)[
^])
The first class has to have the partial keyword or the compiler won't know that it should look for other parts of the class before closing it.
If you don't have the ability to change the first part, then you need to have a new class inherit from the first class. Then you can add as much as you want and still have access to the original parts.