Basically our client has 2 PIX configured as Active/Standby, they decided to replace the devices with ASA. I found out that the existing PIX has 6 interfaces; 1 in, 1 out, 3 DMZs and 1 FO. The ASA that my company supplied was 5520s with just 4GE interfaces and 1 mgmt. What is the best possible solution to complete the migration without adding any module. Is it possible to create a subinterface on one of the physical interface and trunk it?
Yes, configure vlan subinterfaces and assign different security levels to the subinterfaces and you'd do this by setting the link from the switch to ASA as a trunk. Try to keep the outside and failover interfaces on a dedicated physical interface, if possible.