The easiest way to get any code in Word is to use macro-recording. Set on recording, do what you want, and you have your code:

Standing in the open document.
Code:
Documents.Open FileName:="MyFile.doc"  'Opens the other document
  Selection.WholeStory                   'Selects all text
  Selection.Copy                         'Copy text
  ActiveWindow.Close                     'Closes that document
  Selection.EndKey Unit:=wdStory         'Goes to the end of the 1st document
  Selection.Paste                        'Paste the text here
And you're done.