Don't believe any Nissan dealer will purchase a warranty from GM.
As RYD states, make sure it is good at any dealer (at least every Nissan dealer), or you might be screwed on a long trip. You might have to pay, then get reimbursed.
There are aftermarket companies that sell, or at least sold bumper to bumper coverage on everything except body and interior wear items. I had one on a Vette, issued via Warranty Direct.
Make sure the aftermarket warranty is covered by a reinsurance company. Two bigs ones, Warranty Gold and another one (can't remember the name right now) went bankrupt in the past three years.
You can purchase a Hummer warranty from another GM dealership. In fact, sometimes it is a lot less expensive. Fichtner Chevrolet in Montana will sell Hummer warranties, or at least last year they would, at a great cost with no requirement for sales taxes unless you live in MT. I believe Bean Pontiac/Chevy in either NC or SC will also sell a GM extended warranty on any GM vehicle, and the Hummer is just another brand of GM vehicle.
GM warranties are provided by MIC (motors insurance company) owned by GMAC.