Right, so now I'm thinking about how to flood Hell with magma. The obvious problem is that glowing pits function as bottomless holes, so its necessary to either floor over every single bottomless pit in existence or pump magma in with such force and pressure that it flows in faster than it can be drained. Therefore, I recommend the following.
Create fortresses covering every single tile of the world map. Each of these fortresses will have a mega project consisting of a lava reservoir covering at least ten z levels, preferably more, and shall be receiving constant input from the magma sea. Additionally, each of these fortresses shall engage in a military campaign designed to temporarily pacify Hell while the glowing pits can be floored over by our intrepid dwarven masons. Once the flooring is completed, Hell shall be resealed via constructed floors to prevent Hell from being prematurely unleashed. Finally, Each fortress shall have a lever which is easily accessible from the surface which, when pulled, has the effect of opening Hell via cave ins and simultaneously draining the reservoir into Hell. Once each and every tile of the world map is covered in said fortresses, a brave adventurer shall journey across the globe, pulling each and every one of the great Levers of Infernal Retaliationtm. Once his epic task is completed, he will retire a heavily fortified clear glass pyramid so that he may contemplate the glory of the burning world.