Irule for combine useragent & xforwarder-for

I'm not sure I follow.

The F5 passes the User-Agent from the client to the server untouched. The X-Forwarded-For header can be added with a HTTP profile with the X-Forwarded-For option enabled. The server will then receive both items. No iRule is needed.

If you need an iRule to combine both then this will do the job for you.

when HTTP_REQUEST {
  HTTP::header insert "X-Audit-Info" "[IP::client_addr]:[HTTP::header User-Agent]"
}