Adding funds and reopening a closed encumbrance document is not always the best option.  There are times when a new award document should be created to comply with purchasing rules and the procurement code.  If it has been determined that re-opening the closed encumbrance is the correct approach, you can create a modification of the PO/CT document to add more funds, then use the CEC job aid located here to reopen it in order to continue processing payments from the reopened PO/CT.