Complex sentence: That same year, Vaggelis Stamatis, a mathematics teacher and member of the Panhellenic football league, joined Atromitos'administration board--because of his prominence in pre-war Greek football history, Stamatis' involvement increased the new team's stature in Athenian football.
Split into simpler sentences: That same year, Vaggelis Stamatis, a mathematics teacher and member of the Panhellenic football league, joined the administrative board of Atromitos. Because of his prominence in pre-war Greek football history, Stamatis'involvement increased the new team's stature in Athenian football.