Some friends had a house that had had an addition added onto it, naturally at he far end of the house from the air handler. It was pretty easy to fix once we thought about it. Yes it had thrown the system out of balance, because it had added to the house.
The first thing that we did was to wrap the long run of pipe with a nice thick coat of insulation. That pipe was loosing a lot of cool running that far in a hot attic.
The house was in Florida and didn't have ridge vents, so we ran on the length of the roof. Attics in Florida get hotter than blazes, so we also threw in a thermostatically controlled fan and blew in insulation to the level it needs to be in Florida if you don't. A little of the track, but if your going to be up in your attic have a look around. I bought a house to revamp and about had a heart attack when the first power bill showed up. I climbed up into the attic that was just high enough to crawl down the middle up and was very surprised to find it getting cool as I neared the far end of the attic. I found that the plumbers that had re plumbed the house for they elderly lady that had owned it had leaned over the first duct leading out of the main to connect the water. They had bent it down and there was a 4 inch wide opening where the first duct and main met. They let that lady pay to air condition her attic for a lot of years.
Then we put in an air activated booster fan into the duct that lead to the addition. When the A/C kicked on the fan detected the air movement and turned itself on. The fan was adjustable for the amount of extra cool air it needed to pull to keep the addition as cool as the rest of the house. I would think that they all are. If you didn't pull enough the room would still be warm and if you pulled way to much that room would be freezing at the expense of the rest of the house.
I'm about 99% certain that I've seen the booster fans that screw over vents that would really make it easy. I wouldn't use one that you couldn't adjust the air flow. You might get lucky or you might throw the who house out of wack. We wrapped the duct in hopes of avoiding that.