Yes. For outbound, configure the FXO voice grouped-trunk with the same patterns as the PRI but a higher cost.
For inbound, do the following:
On your FXO trunk add the line:
where "3015550110" is the extension on the PBX of your auto-attendant or whatever destination you want FXO callers to reach. This should be whatever digit length you have in the accept pattern of the PBX's voice grouped-trunk. Ten digits for example here.
Note that inbound calls that ring the FXO will route to the PBX all the time, regardless of whether the PRI is up or down.