No need. I think I get it. Try this on your word document. Ignore the spreadsheet for the moment. See if you get what you want. Use Ctrl-Z to undo if necessary.
Code:
Sub expand_bookmark_to_include_insertion()
Dim bk As Bookmark
Dim new_range As Range
Set bk = ActiveDocument.Bookmarks(1)
Dim bk_name As String: bk_name = bk.Name
Set new_range = ActiveDocument.Range(bk.Start, bk.End)
new_range.InsertAfter (vbCrLf & "Inserted text")
bk.Delete
Dim bk_new As Bookmark
Set bk_new = Bookmarks.Add(bk_name, new_range)
End Sub