Adding Noise in Frequency Domain vs Time Domain
I usually like to add noise with my own code.
Adding Whit Noise is really easy task given the signal as you have.

All you need is to calculate your signal second moment at the frequency and add noise to the frequency bins such that the second moment of the noise creates your desired SNR.

Since the DFT is unitary transform, adding white noise at frequency domain is equivalent to adding noise at time domain. 

There 2 options here, are you looking for adding White Noise (Namely spread all over your frequency) or Band Limited (Colored) noise?

For white noise just add Noise to each bin in the frequency domain such that the noise Second Moment (The Means squared multiplied by the number of bins) is as your desired SNR.

For colored noise do the same but only for the relevant frequency bins (Where the second moment is calculated only on the number of "Active" bins).