How can I shift the frequency of a whole audio file. So as if there was a sound at 500 Hz and I wanted to shift the audio by 50 Hz, that sound would then be at 550 Hz, and a sound at 1000 Hz would be shifted to 1050 Hz?
That operation is indeed called frequency shifting. It can be achieved with a Hilbert transform. How is described in this article – pretty easy to code it up yourself in any DSP language of choice1, but I'm not aware of any tools that offer this as a standard operation.
The thing is, frequency shifting is a pretty weird operation. For audio purposes, it is quite reminiscent of the typical ring-modulation character (which isn't entirely surprising, since it is implemented with multipliers). At any rate, if you frequency-shift, say, human voice, up a few Hz, it'll sound nothing like the same voice speaking/singing a tone higher, because the harmonics don't match anymore. Are you sure what you want is not pitch shift (which multiplies all frequency by some factor)? That's available in many free or commercial implementations, e.g. Rubber Band.
1Much easier than pitch shift.