Apache with PAM Authentication Fallback

I have Apache 2.2.x running on FreeBSD 7.2

I currently have Apache prompting for a username and password which is read from a text file, while allowing a certain IP addresses to access a site without logging on.

I would like to retain this function, but add in the ability to have Apache fall back to the system's user accounts (through PAM) if the user does not exit in the text file.

EXAMPLE 1:
pperson is a system user
pperson is NOT in the .htpassword file
pperson is NOT in the allowed IP range
pperson can log on to the website using his FreeBSD username and password.

EXAMPLE 2:
ssomeone is a system user
ssomeone is in the .htpassword file
ssomeone is NOT in the allowed IP range
ssomeone can log on to the website using the username/password in the .htpassword file.

Below are my current Apache directives

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order allow,deny
AuthType Basic
AuthName "Private Access"
AuthUserFile /usr/local/etc/apache22/.htpassword
require valid-user
deny from 192.168.0.37
allow from 192.168.0.0/16
Satisfy any