Hi,
You wont be able to use the PIX configuration directly in the ASA5525-X model as they are too far apart in software levels to be compatible.
The ASA5525-X only supports software levels of 8.6 and above while the PIX probably goes up to 8.2 if I dont remember wrong.
Though at the same time I have to say that a lot of configurations have stayed mostly the same but with regards NAT and ACL configurations there are major changes. The amount of manual work converting the PIX configuration naturally depends on how complex/large the configuration currently is.
Here is a document I wrote about the new 8.3+ Software level NAT configurations in 2013
https://supportforums.cisco.com/document/132066/asa-nat-83-nat-operation-and-configuration-format-cli
The following configuration might be of great help to you if you convert the current PIX NAT configurations
https://supportforums.cisco.com/document/33921/asa-pre-83-83-nat-configuration-examples
Here is also an official Cisco document related to the new software levels.
http://www.cisco.com/c/en/us/td/docs/security/asa/asa83/upgrading/migrating.html
The big changes regards NAT and ACL changed when ASAs moved from software 8.2 to software level 8.3 and beoynd.
Hope this helps :)
- Jouni
