You shouldn't alter a garage door at all. If you change the weight of the door, you will mess up the counterweight.
Many people do not realize this because they've never lifted a garage door (the opener does all the work)... but a garage door is balanced. You should be able to open one, certainly a residential one, with one hand, and if you move it to a height, it should stay there.
That is where spring rate comes into play. The door must be "balanced" (weight of the part of the door that is on the vertical equals spring tension) through the entire range of motion. You have some limited range of adjustment on the spring. But go too far and you get a tilted spring rate: the door wants to spring upwards when near top, and drop when near bottom. Or the reverse. That is when you need a different rate spring.
Garage door insulation kits exist, but they are designed to be of trivial weight.
Garage door safety is serious business, and the counterweight stores a lot of energy. It's most dangerous to people trying to service the counterweight mechanism. It's less about the spring grenading, and more about the door making unexpected movements, in particular, unexpectedly dropping.
I recall one building where some epsilon-minus designed the opener to act on the spring shaft instead of the door. Terrible design. Left free to move, the cables, drums and spring shaft are fast enough to keep up with any door movement. Not in this case. When the door is all the way up (stored horizontal), it is lazy about coming back down. Turning the counterweight shaft didn't move the door, it only slacked the cables, which is a cardinal sin because then, the cables pop off the drums. When gravity finally took the door, it dropped like a guillotine. Doors kill.
If you want a better insulated window door, figure out who made your door and consult with them about available options. They may sell door panels that do what you want cheaper and with less of your time than you could build one. More importantly, they could consult with you about any spring changes that would be needed. Springs are deadly and are not to be trifled with.