No, LPG in the UK is Liquified Propane Gas whereas CNG is Compressed Natural Gas. The concerns in the link you gave seem to revolve around the volume you would need to sacrifice to accomodate the CNG and also the explosive nature if the tank punctures. With LPG this is less of an issue as LPG is much denser than CNG and also being a liquid it won't immediately explode if the tank is punctured. The other major benefit is that the energy content per kg of LPG is much greater than CNG. As a matter of interest, in the UK today the cost/mile of LPG versus petrol is about 2/3rds - hence the interest.
With Ampera you have no room for an LPG tank under the vehicle and many LPG vehicles utilise the spare-wheel-well under the boot floor - any you don't have one of those either (!)
If you managed to conver the car (somehow) and then had an range extender issue (which you would after a little time) you would have zero warranty on your RA.
We did look at BioEthanol, but additional engine production cost and a lower range. Bioethanol hasn't got the calorific value of petrol for the same volume. Plus, Bioethanol fuel stations are rarer than public ChargePoints, so you'd be using petrol on long journeys. Therefore no point in charging customers for something they could not use.
Both of my previous cars were LPG and I've read and spoken to a lot of people about it.
The short answer is yes, the Volt/Ampera can be converted to run on LPG. There's no reason why it wouldn't work (although I wouldn't want to be the engineer that has to find somewhere in the engine bay to mount the control box and regulator!). The fuel tank would have to go in the boot though, so you would lose boot space. None of the warranty should* be affected unless you have an issue which is directly in relation to the conversion - in which case the garage which did the conversion would be responsible for that.
However, I would seriously doubt it would be be financially viable. The first car I had converted was a PT Cruiser which cost almost £1000 to have the LPG fitted. Due to the inefficiency of that car (~28mpg) and the distance I was driving, it paid for itself after 9 months, so after that it was cheap(er) motoring. It made obvious sense to me.
But for anyone that uses their Volt mainly on battery, the payback time is going to be huge. How long will it be before you've used £1000 worth of petrol? That's how long it will take until you see any financial savings.
* Dealers will try and use any excuse not to honour a warranty, and seeing LPG fitted makes them think it's a free pass. "Wheel bearing excessively worn after 6 months? Sorry mate, you've got LPG so no warranty!". It wouldn't stand up in court though. (disclaimer - I am not a lawyer)