A low signal modulation format-enabled highly spectrally efficient new transmission technique termed subcarrier index-power modulated optical OFDM with dual superposition multiplexing (SIPM-OOFDM-DSPM) is proposed and investigated, for the first time, in intensity modulation and direct detection (IMDD) passive optical network (PON) systems incorporating directly modulated DFB lasers (DMLs). The dual superposition multiplexing (DSPM) operation passively adds two low signal modulation format-encoded complex numbers and assigns the sum to subcarrier index-power modulation (SIPM)-generated low and high power subcarriers. Detailed numerical explorations of SIPM-OOFDM-DSPM performances over additive white Gaussian noise (AWGN) channels are undertaken, based on which optimum key transceiver parameters are identified. In comparison with previously published techniques, over simple DML-based IMDD PON systems, SIPM-OOFDM-DSPM considerably improves the signal transmission capacity, system power budget and performance tolerance to both chromatic dispersion and fiber nonlinearity. The work suggests that SIPM-OOFDM-DSPM is a promising signal transmission technique capable of providing cost-sensitive PON systems with improved transmission capacity, performance flexibility and adaptability.