I am not sure about the USA and the requirements to register as a psychologist. But, in Canada, you NEED an undergrad in psychology, typically an honours, but you can get in without one if you have completed other major research projects. The reason a psychology degree is required to be accepted is because you are required to have an undergraduate degree in psychology to register as a psychologist within Canada as part of the Canadian Psychological Association requirements--which is based on the APA so it may very well be the same. Again, I can't say its the same in the states, but I would look closely into that because you have need to do an undergrad in psychology to go on the clinical path.