I think that you've confused two different things:
Supersampling (a.k.a oversampling) is the process of increasing signal time-resolution (you might say: horizontal). It multiplies the sampling rate of a sampled signal by adding extra samples between existing ones with interpolated values. This allows for higher-precision processing further the road and helps minimize some processing artifacts. This process applies only to digital signals, because analogue signal isn't sampled, it's continuous. One could say that an analogue (voltage) signal has infinitely high sampling rate, but that's not technically true, it's just a figure.
Dithering is adding noise to increase the dynamic (vertical) resolution of the digital signal that is about to be quantized. Quantization is necessary to store samples in finite precision numbers (digital files). Adding noise before quantization replaces the quantization distortion that produces audible artifacts called quantization distortion, with much less audible noise floor.
You can't increase sampling rate (and therefor frequency range) by adding noise to the signal, but you can increase the dynamic range by replacing quantization distortion with it.