Dynamic HTTP Header Addition

["Norbert Naveen" <norbert.naveen@xxxxxxxxx>]

Hello ,
   There is possibility of adding a new HTTP header via
    Usage:   request_header_add field-name field-value acl1 [acl2] ...
    Example: request_header_add X-Client-CA "CA=%ssl::>cert_issuer" all

   Wanted to know if the HTTP header value can be dynamic ? and can invoke 
a program to fetch the Value dynamically and populate ?
   If so how can the same be achieved.

Thanks in advance
Rgds
Norbert