A Turbo Trellis-Coded Modulation (TTCM) aided superposition modulation scheme is conceived for a Decode-and-Forward (DAF) based cooperative communication system. More specifically, two source nodes communicate simultaneously with the same destination node via a relay node. Superposition modulation is invoked at the relay node in order to combine and simultaneously transmit the two source signals to the destination node. Hence two timeslots are used to transmit two source signals. Extrinsic Information Transfer (EXIT) charts and power sharing techniques are employed in our design. The performance of the proposed scheme is investigated for transmission over uncorrelated Rayleigh fading channels, which is within about 2~dB of the corresponding capacity.