Adding white noise to complex signal (complex envelope)
If you want a complex white noise sample with variance $N_0$, then you should generate two independent noise samples each with variance $\frac{N_0}{2}$. The two samples that you generate make up the real and imaginary parts of the desired complex noise sample. The resulting complex value will have the desired variance $N_0$. You should then sum this with your complex baseband signal to get the effect that you want.