To make a QFR backlit you need to drill holes in the PCB and do some serious hacking. I'm not sure if it is even possible, it depends on the PCB. Are you sure you want to do that?
I have a hacked on / broken several times and repaired QFR PCB on hand and I will say that it would be easier to replace the PCB entirely. If course, modding the original PCB is possible, but you'd have to hand wire the LED matrix yourself, drill a lot of holes with a paperclip, and then repair traces. It's possible, but pretty difficult. Some may consider to to be more trouble than it's worth.
Whatever you decide on the DIY route, we are here to help!