Partial classes in separate dlls

From MSDN -Partial Classes and Methods:

All partial-type definitions meant to be parts of the same type must be defined in the same assembly and the same module (.exe or .dll file). Partial definitions cannot span multiple modules.

No it is not possible. When the assembly is compiled the class needs to be finished.

No. Partial classes are a purely language feature. When an assembly is compiled, the files are combined to create the type. It isn't possible to spread the files out into different assemblies.

Depending on what you want to do, though, you might be able to use extension methods to accomplish what you need.