There is no direct way how you could ever convert crypt8 Whatsapp messenger message archives to db.crypt5 format, because they are of different version.
However, the WhatsApp Tri-Crypt program can be run as background process and convert all newer crypt8, crypt7 and crypt5 archives to the original legacy .crypt format, which then can be easily extracted with one of the available tools like WhatApp Xtract.