You might try something along the lines of:
Sub Demo()
Application.ScreenUpdating = False
Dim DocSrc As Document, DocTgt As Document, RngSrc As Range, RngTgt As Range
Set DocTgt = ActiveDocument
Set DocSrc = Documents.Open("Z:\xxx.doc")
Set RngTgt = DocTgt.Bookmarks("Destination").Range
With DocSrc
Set RngSrc = .Range(.Bookmarks("Start").Range.Start, .Bookmarks("End").Range.End)
RngTgt.FormattedText = RngSrc.FormattedText
DocTgt.Bookmarks.Add "Destination", RngTgt
.Close False
End With
Set RngSrc = Nothing: Set RngTgt = Nothing: Set DocSrc = Nothing: Set DocTgt = Nothing
Application.ScreenUpdating = True
End Sub