Yes they can. I used hard cases, for future trips I will change to soft luggage though because everything will be ground to dust in hard panniers if you don't pack tightly and ride hard off-road for days. Walter Colebatch designed the "Magadan" soft panniers after riding the Baikal-Amur Mainline, I personally haven't used them yet but will try them for the next trip. If you're into this kind of stuff, check advrider.com.
Edit: Can those lighter bikes take panniers? I guess you're not going a long way with a rucksack?