You can use any unused pin to input en external clock.
At first, add a digital input pin with a short wire and name the signal name of the wire.
In this case the input signal is declared as "EXTCLK"
Then open the clock editor from the CYDWR file.
In the "Digital Input" box you can specify the signal name which is used for a clock via the DSI. The Digital Input clock can override the IMO clock and is used for the PLL reference clock.