These are my ingredients for a one-loaf white bread recipe:  3 cups all-purpose flour 1 cup milk 4 Tbsp butter 2 Tbsp sugar 1 1/4 tsp salt 1 packet active dry yeast  I'd like to add an ingredient that will increase the loaf's fiber content without having to adjust or alter the other ingredients, but I don't know a) what to add, and b) how much of it to add.  
You can add a handful of pumpkin seeds or sunflower seeds. You don't need to adjust any of the other ingredients.  Sunflower seeds contain a ton of good stuff, including fibre, according to Wikipedia:  "In addition to linoleic acid (an essential fatty acid), sunflower seeds are also an excellent source of dietary fiber, some amino acids (especially tryptophan), Vitamin E, B Vitamins (especially vitamin B1 or thiamine, vitamin B5 or pantothenic acid and folate), and minerals such as copper, manganese, potassium, magnesium, iron, phosphorus, selenium, calcium and zinc.[5] Additionally, they are rich in cholesterol-lowering phytosterols." 