Automatically would be a stretch, but you can use a macro to create a tab at the right margin and add a tab with a dashed leader to fill the space. Put the cursor in the 'line' in question and run the macro
Code:
Sub TabToEndOfLine()
Dim lngWidth As Long, lngLM As Long, lngRM As Long
Dim oRng As Range
With Selection.Sections(1).PageSetup
lngLM = .LeftMargin
lngRM = .RightMargin
lngWidth = .PageWidth
End With
Set oRng = Selection.Paragraphs(1).Range
oRng.End = oRng.End - 1
With oRng.ParagraphFormat.TabStops
.ClearAll
.Add Position:=lngWidth - lngLM - lngRM, _
Alignment:=wdAlignTabRight, _
Leader:=2
End With
oRng.Collapse 0
oRng.Text = Chr(32) & vbTab
oRng.End = oRng.Paragraphs(1).Range.End + 1
oRng.Collapse 0
oRng.Select
lbl_Exit:
Set oRng = Nothing
Exit Sub
End Sub