I finally stumbled upon the correct configuration guide which showed me exactly what I need. Simply use the "DNIS Substitution" feature. I thought this feature would only work with inbound calls not outbound...I was wrong.
It is applied at the SIP Trunk account level and it successfully adds the 5-digit account code for both calls from the PRI and from the analog station ports on the TA904.
Refer to this:
Custom Trunk Group Access Configuration Guide
Trunks > Trunk Accounts > (click on the appropriate SIP trunk name) > DNIS Substitution (tab)
Match Number: NXX-NXX-XXXX
Substitution Number: 12345-NXX-NXX-XXXX
voice trunk T01 type sip
description "your SIP trunk"
match dnis "NXX-NXX-XXXX" substitute "12345-NXX-NXX-XXXX"
match dnis " 1-NXX-NXX-XXXX" substitute " 12345-1-NXX-NXX-XXXX "
match dnis "011-$" substitute "12345-011-$"
match dnis "911" substitute "12345-911"