I found the solution to be very simple.
Once the document has been parsed, the following will generate an Enveloped Signature (as specified here):
// rootelem contains the root element of the parsed document
XSECProvider prov;
DSIGSignature * sig;
DOMElement * sigNode;
sig = prov.newSignature();
sigNode = sig->createBlankSignature(xercescdom, CANON_C14N_COM, SIGNATURE_HMAC, HASH_SHA1);
// append the signature node to the document's element which is being signed, here
// it is the root element
rootelem->appendChild(xercescdom->createTextNode(MAKE_UNICODE_STRING("\n")));
rootelem->appendChild(sigNode);
rootelem->appendChild(xercescdom->createTextNode(MAKE_UNICODE_STRING("\n")));
// create the envelope reference and the signing key (e.g. HMAC Key)
// set the signing key
sig->setSigningKey(hmackey);
// other steps... Serializing the rootelem will generate an XML document with Enveloped Signature
The following will generate an Enveloping Signature:
XSECProvider prov;
DSIGSignature * sig;
DOMElement * sigNode;
sig = prov.newSignature();
sigNode = sig->createBlankSignature(xercescdom, CANON_C14N_COM, SIGNATURE_HMAC, HASH_SHA1);
// append an "Object" element to the signature object
DSIGObject * object = sig->appendObject();
// in an enveloping signature, the "Object" element contains the data being signed
// so the rootelem can be appended as a child to this object element
object->appendChild(rootelem);
// AND you are done!
// now create the envelope reference and the signing key (e.g. HMAC Key)
// set the signing key
sig->setSigningKey(hmackey);
// Serializing the signature node (sigNode) will give you the required XML with Enveloping Signature.
Similarly a Detached Signature can be generated with some effort.
The above examples cover very simple cases. A little bit of effort will be required for signing multiple data items and subsets of document.