When you use the GetActiveObject Method, it is getting the current instance of Excel.
If you want to do a cross program GH component, then I suggest you to reference DLLs : Microsoft.Office.Interop.Word and Microsoft.Office.Interop.Word.
Then you can do whatever you want with Word/Excel from wherever you want, including GH.
To get it started, just create a new instance of Interop.Word.Application f.ex., it will start the program if not already opened. I should have both SDKs somewhere if you cannot find them.