Answer:
Most possum species are found only in Australia (the possum is a completely different animal to the opossum) or neighbouring New Zealand, where it has been introduced. Australia does not have rabies, so therefore it is quite safe to say that Australian possums cannot have rabies, and therefore cannot transmit rabies to dogs.
Some people believe possums are the same as opossums. They are completely different species. But if the question is in regard to opossuma: whilst it is believed that any warm blooded mammal can get rabies, opossums' body temperatures are generally too low to harbour rabies. It would be extremely rare to see this, but there has been at least one documented case of a rabid opossum (see the related link).