First- be specific about what you want, exactly how you want to connect Asterisk to your PBX, what functions it should handle, etc etc.
I assume you want to use asterisk as a sort of glorified ATA to add Internet telephony features to your existing PBX so you can save money on your phone bill. by routing calls through a voip provider instead of your local telco.
This is quite possible and relatively easy to do. First, you need to get calls out of the PBX and into *. The best way to do this might be with a PRI interface between the two, although that’ll be a bit pricy. If your PBX has some analog (pots) FXO ports (that you connect to the telco CO and phone line) you can use those. However, in such a case if you dont want asterisk to do anything fancy like play IVR menus, you might be better off with a few Sipura SPA3000 adapters, as they can do failover to POTS for 911.
As for hardware, you’d probably need (for asterisk) one or more TDM400 cards from Digium. The TDM400 is a 4 port card that accepts 1-channel modules, each module activates one of the 4 ports as either FXO (red module, connects to analog phone line) or FXS (green module, connects to analog or ADSI telephone adn provides dialtone)
Asterisk, via extensions.conf, can very intelligently route calls based on pretty much any criteria. Sending 911 calls to POTS is not a problem