The demand for converting Hummers hasn't been enough to warrant developing and testing a kit, so there isn't one available. The only way you could do it is to have an LPG system supplier do the work using your car as a test bed. Sometimes they will do the extra work at no cost to you if they feel they would be able to sell kits to other owners. In that case you would just pay the normal fee for conversion, but it's unlikely they would offer that service for a Hummer as the on-going demand is likely to be very low with little chance they could recoup the cost of the development and testing they would have to do.