Is there a way to add a ultrasonic sensor on the ev3 COR3 robot?
The ultrasonic sensor is useful for navigation.  This robot design has a nice skirt and positioning the ultrasonic sensor should not interfere with that skirt.  Also the ultrasonic sensor is excellent at detecting right angles like the floor meeting a wall.  So, I'd recommend positioning the sensor on either a side (for following along a wall) or the back (to slow down just before reaching a wall when backing up).  It doesn't matter a lot whether the sensor is mounted vertically or horizontally.
So, given all that how do you mount it on the robot?  Pins and a couple beams or right angle beams.  You might also find an existing small pin that can be replaced with a deeper pin.  Maybe the better answer is lots of trial and error.