Even among insiders, only certain people can get inside the facility, so it's even more impossible for outsiders to get in.
この場合、次のような言い方ができますよ。
ーEven among insiders, only certain people can get inside the facility, so it's even more impossible for outsiders to get in.
「内部者でも限られた人しか施設の中には入れないので、外部者が入るのはもっと不可能だ不可能だ」
insiders「内部者」
to get inside「中に入る」
facility「施設」
outsiders「外部者」
ご参考まで!