How to generate AWGN with given power spectral density
Here is a Matlab snippet that might help. You should be able to apply the result to your favorite PSD routine to see the dbm/Hz in the frequency domain.

% create complex noise with a desired dBm/Hz
fs=1e6;             % whatever you want
N=1e5;              % # samples of noise
R=50;               % ohms
dbm_hz = -135;      % goal
bw = fs;            % complex noise with BW = sample rate
dbm = dbm_hz + 10*log10(bw);
dbw = dbm - 30;
watts = 10^(dbw/10);
vrms = sqrt(watts * R);
n = randn(N,1) + j*randn(N,1);
n = vrms * n / std(n);
meas_dbm_hz = 10*log10(sqrt(mean(n.*conj(n)))^2 / R) - 10*log10(bw) + 30