Can you extend rake edge on existing gable?
Structurally it will not be an issue at all, as long as the overhang isn't huge. Up to a foot is good for adding on as long as you can get 2' back into the existing rafter (3' total). If the proposed overhang is shorter, that will not allow you to use a much shorter piece to tie in. At least for me it would not, the reason being, too short of a piece, with stress applied in one fashion or another, will readily split. With care on assembly, (predrill, screws and glue)you may be able to reduce the tie in to 18" with a 6" overhang (24" total)

Of course, the larger the tie in the better, but you don't need to go overboard. All things considered it will be easier to do this when the roof is to be replaced, that way when you do the tie in, you can remove a little of the sheathing an do the fastening with ease. Then the sheathing can be replaced all the way out to the new drip edge.

The last reason you may wish to wait until the reroof, is based on my experience and math. Roofing lasts a minimum of 20 years, at least hopefully. You figure that there is 8 years of life to it at the most. When the roof is over 10 years old, in your case 12, at least, the careful pulling of the fasteners holding the existing shingles to add the new ones under them, will be taking a big chance. This also depends on the adhesive that is particular to the manufacturer of the shingles that hold one shingle to the next. On my old roof, before I replaced it, and many I worked on, the sealant will separate readily in the cooler seasons, but would rip up in the summer, crack in the winter. On my new roof, I had to make a repair a few months after I installed it. Thinking cooler weather was needed, I tried it then, but this sealant was different than before. I found that summer heat was needed, and with great care to remove the course of shingles to make the repair. A heat gun with a light touch helped too. As a reminder I bring this up because you will need to separate the courses of shingles to remove the starter course to add more over the newly extended overhang.