Designed proximity sensor in CY3280-MBR3 kit can detect proximity up to 10cm.
You can test increasing proximity sensor distance with CY3280-MBR3 kit temporarily, if you make a prototype sensor.
Following the AN64846 Section 3.6 Proximity Sensing Design, make the proximity loop sensor with copper tape and connect this sensor to PS1 pin(J2 connector) in MBR3 evaluation kit, then you can find the proximity distance as you wanted. (Maximum proximity range is 30cm.)
What is the background of your application?
You may know that if the proximity distance is 30cm, the sensor would be very sensitive. Depend on the application background, the sensitivity could lead to larger noise.
It's been mentioned that using proximity feature we can achieve low power consumption. So I want to do that. But I am not getting how to achieve larger proximity range. Do I have to design separate pcb design using both proximity electrodes? Do I have to go with trial and error method?
You need to wire up a proximity sensor to the capsense. You can plug in a custom proximity layout/circuit design to get more range/sensitivity by following the appnote that szin linked. Page 9 has the start of the section detailing proximity circuit layout/guidelines. The document also describes what/how/why to use proximity sensing in detail.
In answer to your direct questions: Yes, you will need a separate PCB/proximity circuit connected to the electrodes.
You can design the circuit with theory only, but there are enough random variables that you will want to test your design and tweak physical setup to maximize your design constraints.