Emran:
You can add external code memory to your microcontroller. With most parts you can fit up to 64K bytes of read only type storage. It is typical for parts that have onboard flash that that space will subtract from the total available external code space.

As you say,,,it is also possible to add 64K of external data memory. For your application this is what I would do. The external data memory can easily be up to 64K of memory. This could be an EEPROM that you program in an EPROM programmer prior to mounting on your board. On the other hand if you used a FLASH chip you could use it in a non-volatile manner and yet support reading, writing, and erasing of the flash chip.

In the Keil C, lets say you provided a 32K byte external EPROM memory as a data memory, fitted at addresses 0x8000 to 0xFFFF, then you could define an array in this space similar to:

unsigned char xdata lookup_table[32768] at 0x8000;

Good Luck
Michael Karas