I've installed many 16x7 doors that had a single spring. Actually, I converted an old one that had two small springs to one larger single spring. It was all I had on the truck and the customer needed their door fixed that day. The door balanced almost perfectly throughout it's entire travel.
There's a science to it, it's not just a simple matter of adding an extra spring. The length, wire size, and coil diameter are all calculated factors in getting a properly-sized spring. With the wrong spring, the door could be heavy the first few feet of travel, then super light the rest of the way, or vice versa. Either way, it's dangerous and will put much undue stress on your operator. Depending on how much weight is added, you may be able to just have a the springs wound a little more to compensate. In fact, it may actually balance better than it ever has. I've balanced doors that were fairly good throughout the first few feet of travel, but then got really heavy before the last section or two was in the horizontal track. This made it difficult for the operator to pull the door up. Adding more tension made the door light at the bottom, which made the operator work harder to push the door down, and could bend the top section over time. I added tension, then countered that by adding extra struts to the top and fourth sections. If all else fails and you need to change the spring, you're gonna have to contact a door company with the size and weight of your door so they can get you the best spring possible. There's a relatively easy way to weigh your door that involves a bathroom scale and various lengths of 2x4s:
