1. No you can't.
so if you're interested in reducing the running cost of large-engined cars, and you plan to keep the vehicle and not turn it over every 2 years
2. Replace your diesel vehicle with petrol and
3. Have PLG kit retrofitted.
From personal experience, a 4L engine in a heavy saloon is giving me 40mpg in cost terms. It will have paid for itself in 16000 miles (ish)
This chap specialises in converting yours, or selling you one that has been done. He's thoroughly reliable.
In fairness to the above posters they are wrong. You can have LPG fitted to a diesel engine. But it doesn't work in the same way as LPG on a petrol engine. In that instance you run on petrol, when you switch to gas you use all the petrol avail and then run solely on gas and then vice versa when going from gas to petrol.
When using LPG on a diesel engine it is used as an additive. It is allowed to enter the combustion cycle and makes for a more economical burn of the diesel fuel. It can't be used as a stand alone fuel on a diesel system.
It is a lot more expensive to fit to a diesel engine over a petrol engine. I had a few V8's converted to LPG. I ended up fitting my own kit for less than 300 all in.