That is not possible. Once you renew your passport, your previous one becomes invalid, even if it is not yet expired. Assuming we are talking about a US passport, yes, the Passport Agency mails it back to you as a keepsake, but it is stamped "CANCELED" and the MRZ has holes punched in it. The airlines will no longer swipe it to issue boarding passes. It would cause BIG problems if you were to try to enter HK using an expired passort, even if the barcode were still attached to the back.

I think the best option is to do what Fred suggests--wait in line the first time, get processed manually with the new passport, and then go to the eChannel enrollment centre. Perhaps bring the old expired passport as proof that you are simply renewing eChannel membership.