Is there any way to use Flash as a cade & data memory as well? so that i can replace expensive NVRAM used (Or EEPROM& SRAM)

Absolutely except fot the NVRAM and SRAM; however I recommend against using same flash for code and data. It is, indeed possible to so, but there will be many issues to resolve that makes it not worth the while unless you are about to produce a million units. one of my current units uses the internal flash for program and has 256k of data flash for semi-permanent storage and 32k RAM for temporary storage.

Have fun,

Erik