You can access the Range object from a bookmark.
Code:
Sub add_text_to_bookmark()
Dim bk As Bookmark
For Each bk In ActiveDocument.Bookmarks
Dim r As Range
Set r = bk.Range
r.InsertAfter = "Insert here"
Next
End Sub
Also, see this
link. Toward the end there's an explanation for expanding the range of the bookmark. Essentially, to do this, you have to delete the original bookmark and recreate it with a new range.