How to combine bins of my DFT
There are different methods for doing what you want.
But first i have to tell you power spectrum density (PSD) is real and positive and you have to obtain it from squared FFT of signal or from FFT of signal's auto-correlation (FFT of auto-correlations is always positive).
To reduce noise you could apply some damping window on your signal. This method is called modified periodogram.
Also you could divide your signal to smaller segments and find PSD of each segment then find average of PSD of each segment. This method is called Bartlet method. 
Also your suggested method, summation of adjacent bins or applying moving average works too (you must apply it on PSD).