yeah it looks like there are powder mixes available:

http://www.amazon.com/Sun-Luck-Sweet-Sour-Sauce/dp/B0007GD4MI

I looked up a couple recipes and it looks like sweet and sour sauce is usually: sugar, vinegar, cornstarch, soy sauce, and ketchup (sometimes no ketchup and sometimes with pineapple juice). I was thinking vinegar would be the hard part backpacking, but apparently not:

http://www.firehousepantrystore.com/4whtvinpw.html