Yes, you can configure the PIX to allow IPSec(VPN) traffic through the PIX.
You need to allow ESP (IP 50) and UDP 500 ports for the end-to-end devices.
Here is an example, look at the PIX configuration, allowing IPSec traffic for the end-to-end IPSec devices;
http://www.cisco.com/warp/public/707/ipsecnat.html
Hope that helps.
Regards
Yusuf