Adding 1/2 inch extra length to each wing tip
Sure you could but why. Will the length be 1/4 scale. Are you trying to meet an IMAA measurment. You can simply cut off the wingtips cut new outborad ribs out of 1/4" or 1/2" balsa. Lighten them with some holes, glue the wingtips back on and recover. I dont think you will see any difference in flying charechteristics