If Item.CID=838 Then
item.number = item.number + "-" + item.option["Drop Rod Diameter"].value
EndIf

I assume you know that there is a setting in the options tab of the hanger "Add Rod Size" set it to yes and it will export the hanger rod size to the trimble file.  It is not part of the hanger number but is part of the Description which is visible by the trimble operators.