My MCU(ATMega328) configured to get clock from ext. oscillator. Can I still program this MCU without external oscillator using ISP, by just connecting 6 pins to
No. From the datasheet:
Depending on CKSEL Fuses, a valid clock must be present.
This microcontroller needs a running clock to program flash memory, it reads, interprets and validates data received on MOSI line. If you program a CKSEL fuse to use external oscillator, microcontroller expects clock pulses to come from XTAL pins, if you don't provide these pulses, it can not function.
Edit: you can add crystall and caps on a breadboard to program a chip out of the board