Hey g00s3, welcome. I haven't been back here in many years myself. Primary Rate ISDN is a different communications technology so you won't be able to pass that via any IP link.
However, QSIG is the tcp equivalent to ISDN. You can connect multiple systems using QSIG. QSIG can operate over a tcp link such as a vpn without issues. But you need to have a spare PRI interface to do this at office 1 (1 for PRI, 1 for QSIG). Just change the settings for the PRI interface.
2nd option would be to create a H323 trunk to the other PBX. H323 is also a tcp communications layer and simple to setup on the ip office.
However, in your setup, might be just easier to create either a SCN between the 2 sites, therefore needing an SCN license on each site.
Another option is to create a SIP trunk from site 1 to site 2, allowing the other side to dial out aswell as internal calling.
A final option would be to setup a new sip user account on the Adtran box, license and register site 2 to this new sip account. In the Adtran route incoming calls to both PRI and SIP account, allow dialing out from both.
Hope that helps somewhat. If it was me I'd tell the customer to get the SCN license and experience the full feature setup without compromise. It would be a sound investment.
Dave