Yeh, it is one of wishes on the long list members on RCG put together a year ago. We were given hopes, half promisses but to this day it all turned into nothing. Phantom 4 and X4S cameras have both 1" sensors which itself has 3:2 ratio hence this ratio is available, as it utilizes the full area of the sensor. The X5S on the other hand is MFT. As the name suggests the format of the sensor has 4:3 ratio, so the 3:2 format would be a croped area of the MFT sensor. Top and bottom would be croped off. It is possible to automate the process in Photoshop and it is also easy to do manually in LR. Unless you have hundreds of images to crop it should be pretty fast and easy process.
What you can do to assist framing when shooting, is to set custom ratio guides so you will see on your display the 3:2 guides when shooting. That way you can compose your shots accurately.