Assuming the Universality of Computation, then the brain is computable. If it is computable, it is extendable. This would indicate that it is not physically impossible to interface with, offload, and increase computation which currently happens in the brain.
The question then is not whether it is possible, but if it is feasible. To this question there is no known answer, but I for one encourage us to find out.
Clearly more feasible than doom ourselves, with reducing us to machine AI pets...