this is not really going to be on the Asterisk side. The communication to the gateway is through a SIP exchange where the gateway is told to make the call. You need to determine how to configure the gateway to present the proper 9+ pause when making the call out to it, ideally, transparent to the FreePBX setup so you are not forced to require a 9 to dial out.
If you were to use Zap/DAHDI hardware then you could easily achieve this by automatically inserting a 9www into the outbound calls. (where each w= 1/2 sec).
that’s my point, if I understand correctly. It’s a sip trunk to the SPA400 which has fxo ports that interface with the PBX. So it’s the SPA400 that needs to be configured to insert the delay somehow. With an FXO card you could control it directly, assuming that were an option (if the system were physically located next to the AT&T PBX.