Sorry this is a long time replying - just searching for ETRN help myself.
What you want isn't in the package though I don't think. ETRN for the situation where e-smith picks up messages from a primary MX server is there, not for the situation where e-smith is serving mail to a ETRN requesting server.
For this you need to setup e-smith as a secondary mail server for the relevent domain - ie so it accepts mail for the relevent domain and passes it along to the primary when it gets and ETRN prompt from the primary server.
ie setup qmail as a domain seconary, something beyond the scope of e-smith.
But I assume you will have discovered all this already and if you figured it out you must be a qmail guru by now!