Your ISP bootloader usually gets the program from PC and put it in the internal program memory.
If you want to use external memory you must break the harvard memory organisation to a von newman one because you have no intruction to write in the external program memory. So you need a new bootloader.

If you use external memory changing EA you 'loose' the internal memory, so all vectors, codes etc must be in external memory.

Other posibility is to use an for example 8k ROM device, all program memory over 8K must be in external memory, you can redirect all interrupts vectors over 8k to redefine each time you download a new aplication.
You must redirect vectors because you dont know the new address of each routine before write it.

However, why do you want to do this?
You can download the new program version to FLASH and forget all of this!