Seriously? Well, an individual specimen might actually be able to survive, assuming the water doesn't get to cold. But, I'll bet that the Rio Grande's ecology doesn't lend itself to spawning Piranhas. So, any long-term introduction would be unlikely.
But, it's a good thought.
It’s always a bad idea to introduce alien species to a habitat, so please do not put pirahna or any other non-native organism in the Rio Grande. The consequences could be devastating to the whole ecosystem, not just the illegals.