Depending on what you want to achieve, you could mirror and/or page select an area of the external data memory to act as code memory IF you wanted to. In many of the old development boards the external data memory was mirrored into the code space so you could download your code via the serial port and alter it. This was done by 'AND'ing the /RD and /PSEN signals together for the ram /cs signal. So the ram chip would respond on either a code read or external memory read from the cpu.

These days if you application requires bank switching to extend the code space, it may be wiser to look for another cpu that supports a large native address space.