I don't normally supplement with fiber because my kibble meets zoey's fiber needs, but... when zoey has diahrea, I give her pumpkin (which is high in fiber). I bought a big can of plain pureed pumpkin, and spooned it into ice cube holders. Now when zoey's tummy is upset, I will giver her one pumpkin ice cube with a meal.
If you want to add fiber, I would do it with naturally high in fiber foods instead of supplements. you don't want them to have too much fiber or they will be constipated... Some high fiber foods that are pretty dog friendly: Green peas, Sweet potato, brown rice, Corn, carrots, banana...
I personally would probably take a banana, slice it 8-10 peices, and freeze it, then i would give it as treats. I would also use sliced up carrots as treats...