Username and session tracking in an irule

If the application has a unique login page that gets posted to for login you could grab the username, possibly form the query parameter if that's where it is passed, and the session cookie value and stuff them into a different cookie value that you create with your iRule that should persist for the users entire session. Something like this may work.

when HTTP_REQUEST {
    if {[HTTP::uri] starts_with "/loginpage.aspx" && [HTTP::method] equals "POST"}{
        HTTP::cookie insert BigIPsession "[URI::query [HTTP::uri] username]/[HTTP::cookie value aspsessionid]"
    }
}