I think you would have to use scripting, you can use this script from a previous question I had posted.  I think you would just insert your tables and maybe a few more adjustments.

This one I am going from the ROG to write to the SO

 Troubleshooting Loop in SODetail 

This script I am taking the values from the PO to write to the SO, so reverse this thought process, not sure on what you could trigger off of though.

'Script to transfer notes made on PO to SO in UDF field

Dim sPurchaseOrderNo
Dim sSalesOrderNo
Dim sItemCode
Dim sItemCodeCur
Dim sPO_Notes
Dim sSO_Notes
Dim sOrderStatus
Dim oSalesOrder

sPurchaseOrderNo = ""
sSalesOrderNo = ""
sItemCode = ""
sItemCodeCur = ""
sPO_Notes = ""
sSO_Notes = ""
sOrderStatus = ""
sPOSOLines = ""
oSalesOrder = 0

retVal = oBusObj.GetValue("SalesOrderNo$",sSalesOrderNo)
r=oScript.DebugPrint("SalesOrderNo: " & sSalesOrderNo)

if sSalesOrderNo <> "" then

	retVal = oBusObj.GetValue("ItemCode$",sItemCode)
	retVal = oBusObj.GetValue("UDF_PURCHASING_NOTES$", sPO_Notes)
	retVal = oBusObj.GetValue("UDF_SOLINE$",sPOSOLines)

	r=oScript.DebugPrint("ItemCode: " & sItemCode)
	r=oScript.DebugPrint("Notes: " & sPO_Notes)
	r=oScript.DebugPrint("POSOLines: " & sPOSOLines)
	if sPO_Notes <> "" then

		sSO_Notes = sPO_Notes
		
		oSalesOrder = oSession.GetObject("SO_SalesOrder_bus")
		r=oScript.DebugPrint("After oSession.GetObj")

		if oSalesOrder <> 0 then

			Set oSalesOrder = oSession.AsObject(oSalesOrder)
			r=oScript.DebugPrint("After AsObject for oSalesOrder")

			retVal = oSalesOrder.SetKeyValue("SalesOrderNo$", sSalesOrderNo)
			retVal = oSalesOrder.SetKey()

			if retVal = 1 Then
			
				r=oScript.DebugPrint("After Successful SO SetKey ")

				retVal = oSalesOrder.GetValue("OrderStatus$",sOrderStatus)
				r=oScript.DebugPrint("OrderStatus: " & sOrderStatus)

	
				Set oSOLine = oSalesOrder.AsObject(oSalesOrder.Lines)
'				retVal = oSOLine.SetKeyValue("SalesOrderNo$", sSalesOrderNo)
'				retVal = oSOLine.SetKeyValue("LineKey$", sSOLines)
'				retVal = oSOLine.SetKey()

				r=oScript.DebugPrint("After AsObject oSOLines ")

				retVal = oSOLine.MoveFirst()
				r=oScript.DebugPrint("After MoveFirst ")
				sLineSONo=""
				sSOLinesKey = ""
				retVal = oSOLine.GetValue("SalesOrderNo$",sLineSONo)
				r=oScript.DebugPrint("Lines SO Num " & sLineSONo)


				lineUpdated = 0 ' initialize new indicator

				do until (oSOLine.Eof or lineUpdated=1)

					retVal = oSOLine.GetValue("ItemCode$",sItemCodeCur)
					retVal = oSOLine.GetValue("LineKey$",sSOLinesKey)
					r=oScript.DebugPrint("CurrentItemCode: " & sItemCodeCur)
					r=oScript.DebugPrint("CurrentItemCode: " & sItemCodeCur)
																							
					if sItemCode = sItemCodeCur AND (sOrderStatus = "N" Or sOrderStatus ="O") And sSOLinesKey = sPOSOLines Then

						retVal = oSOLine.SetValue("UDF_PURCHASING_NOTES$",sSO_Notes)
						retVal = oSOLine.Write()
						lineUpdated = 1 'trip flag
						r=oScript.DebugPrint("retVal on oSOLine Write: " & CStr(retVal))
						r=oScript.DebugPrint("lineUpdated = " & CStr(lineUpdated))
											
					end if
					retVal = oSOLine.MoveNext()
				loop
							
				if lineUpdated=1 then ' only write if we found a match
								
					retVal = oSalesOrder.Write()
					r=oScript.DebugPrint("retVal on oSalesOrder Write: " & CStr(retVal))

				end if

				retVal = oSOLine.MoveNext()									
						
			end If
		End If
	end if

end if