How to turn a PLC smart meter into 3G or Wifi
Well, you could try to build a device that "talks" the same language as your MCU and UIU and has a cellular modem, but that might, considering that protocol might be very vendor-specific, quite complicated, or even impossible, if they encrypt properly.

Building a simple meter that does exactly what you need might be easier, but that's really up to in-depth analysis.

I have my doubts regarding equipping 300,000 units with cellular internet. These are pretty likely to be located in places with bad/no reception, so you can't cause your customer trouble if you can't reach your device. But: Maybe you want to talk to your regional cellular network operators, they might be planning to roll out an LTE narrowband machine2machine network – LTE has substandards that are exactly for that, sporadic, low rate communications of devices even under bad reception scenarios. With "normal" 2G, 3G, 4G this might get a little hard to make reliable.

Also, the data collection scheme you're describing is the classical example for "why PLC might be relevant in the future"; the power cable is already there, now you "only" need to build PLC units that can talk back to the utility company. 

In fact, such meters exist and are being deployed at large scale in Europe and the US. So, I'd guess, installing these in place of your current meters might be very much cheaper than spinning your own solution.