That makes little sense. SPI has nothing to do with PWM and PMW has nothing to do with SPI. Why not use the SPI pins to control the (mystery) external device? SPI is a bus, that means that the SCK, MOSI ns MISO pis are shared by all the devices on the bus. There is a chip select signal for each device that differentiates the devices on the bus. So, you would have to provide a separate chip select signal* to the mystery device.
*separate and different than the Ethernet shield.
Well the device is a DAC (dac8551). The thing is that i made a pcb with that DAC based on the idea that i will place it on top of the Ethernet shield. I didn't think it right though and i made the pcb to connect to 3 of the Ethernet shield's pins that end up in Arduino PWM pins 9,10 and 11 (on the pcb they are also connected to the DAC pins corresponding to SCLK, MOSI, SS). So i'm asking if i can avoid remaking the pcb. What if i bend the 3 pins connecting to the Arduino PWM and connect them externally to the SPI instead? Would that cause any malfunctions to the Ethernet shield, or are they not used by it anyway?