If I understand your question correctly, you want to configure the PIX Firewall as a DHCP client to a DHCP server allowing the server to configure the outside interface with an IP address, subnet mask, and probably a default route. Yes, you can do this, below is the URL
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/config/pixclnt.htm#xtocid16
PIX can also be configured as a DHCP server, more info in the same URL.
HTH
R/Yusuf
