"Can I connect this pin(pin 31) to one of the port pin and switch it between VCC & GND"

No, for the reason:

Just visiualize that you are switching from internal to external program memory. The instruction in the internal mem would be setb p1.x. The next instruction would be fetched from external ROM and would that a) align on an instruction boundary and b) make sense as the next instruction.
It has been stated that if you have exactly the same instructions at exactly the same location in both memories, you can switch, this, however will only work with derivatives that have identical timing for external and internal fetches.

In short, you may be able to make it work, but you will never be able to make it work reliably and maintainable.

Erik