Redirect user to external URL

You can try and implement this iRule:

when HTTP_REQUEST {
  if { [string tolower [HTTP::host]] equals "www.mycompany.com" } {
    HTTP::redirect "http://www.somewhereelse.com"
  }
}

You will not need any pool for that. Just apply to the virtual server that handles www.mycompany.com. The client will resolve www.somewhereelse.com on his own and reconnect to the www.somewhereelse.com resource.