According the document, user should can access the nParticle data by MFnNObjectData, but I tried several ways to access the data in an nParticle shape node by the MFnNObjectData and MnParticle, none useable. So I have to use traditional MFnDependencyNode and MPlug to obain the data in an nObject node.
I think this part of Maya API should be improved greately, with the AdskShaderSDK, they should be more detailed in future.