How do I add a flag to an alias?
Or, use a function instead of an alias:

con() {
  local OPTIND svr
  while getopts ":ab" option; do
    case $option in
      a) svr=server1 ;;
      b) svr=server2 ;;
      ?) echo "invalid option: $OPTARG"; return 1 ;;
    esac
  done
  ssh username@${svr}.domain.com
}

con -a