Simulated NuSTAR spectra can be generated in XSPEC using the "fakeit" command. First you should download the appropriate response, effective area and background files (which can be downloaded from http://www.nustar.caltech.edu/for-astronomers/for-proposers/simulations). You should then read the background file in as both an XSPEC data file and as the background file, in order to ensure that the BACKSCAL value in the output spectrum is correct. For example:
% xspec
XSPEC12> data nustar_80pct_bkg.fits
XSPEC12> back nustar_80pct_bkg.fits
XSPEC12> resp nustar_rmf.rmf
XSPEC12> arf nustar_80pct.arf
XSPEC12> model whatever
XSPEC12> fakeit
Note that it is important that the "fakeit" command is used (and not "fakeit none" command).