Can I use all channels of a multi-channel ADC to sample an input signal?
Simultaneously sampled means just that. It samples all channels at the same time. As opposed to sequencing through them very quickly. So feeding your one signal to multiple channels won't do anything for your sampling speed. You can do it, but you won't gain anything out of it.
On the other hand, if your ADC sequenced through the channels, then feeding your one input to multiple channels would increase the sampling rate since the sample taken by each channel would be at a different instant in time, and intervals between samples would be closer than if it was just one channel sampling repeatedly.