How to program ATmega with fuses set to 32.768kHz clock?
Even when fused to use a crystal, the ATmega328P will accept a clock signal injected into XTAL1 when being programmed by ISP. The USBasp does not provide such a signal so you will need to set up another circuit to generate a clock signal fast enough for programming.

The programmer should work with such a slow clock assuming the programmer uses a slow clock as well. Verify that the crystal operates normally.