I have had a thought about this. If you use
Merge and Split or
E-Mail Merge Add-in to create the merge, you could run a macro from the process to address the formatting e.g.
Code:
Sub FixID(oDoc As Document)
Dim oRng As Range
Const strFormat As String = "0000 0000 0000 0000 0000 00"
Set oRng = oDoc.Range
With oRng.Find
Do While .Execute(findText:="<[0-9]{22}>", MatchWildcards:=True)
oRng.Text = Format(oRng.Text, strFormat)
Loop
End With
End Sub