Can I use a CEC to reopen a lapsed encumbrance document (PO/CT)? 

No.  The CEC is not designed to reopen lapsed encumbrance documents or commodity lines.  If the encumbrance document is still open (there are commodity lines that were not lapsed), you may add additional commodity lines to the document.  If the encumbrance document was closed by the lapse process then you will need to create a new encumbrance document.