First, it doesn't matter how you whip your egg whites. What matters is how few of the bubbles you created will get popped when you mi in the remaining ingredients. If you use the FP to mix in the remaining ingredients, you'll very effectively break too many bubbles with that rapidly spinning blades. The FP will not continue to make new bubbles while you mix in the other ingredients.
A proper folding process is the best way.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan