Sorry for reviving such an old thread
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.
Spencer
* 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)