MATLAB center frequency shift
You can shift your spectrum in the time domain by multiplying it with a complex exponential. It is the same way you implement an AM modulator. 

Let $A(t)$ be an unspecific baseband signal. You multiply in time domain with a complex exponential and the result will be the bandpass signal:
$$
y(t) = A(t)e^{j2\pi ft} = A(t)*\cos(2\pi ft) + j*A(t) \sin(2 \pi ft)
$$
Since cos and sin both contain only a single frequency component, a fourier transform (if you look at the real part) will result in two impulses at $\pm f$ and the spectrum of $A(t)$ shifted to $\pm f$.

The same applies to your problem: Multiply your time domain signal with a complex exponential of the desired frequency offset and you will observe your spectrum to be shifted down.