How can I fortify homemade soy milk?
You won't be adding any ascorbic acid or such -would curdle the milk- but an oil based vitamin such as found inside a vitamin E gel-cap would mix in fine. Would only be as absorbable or bio-available as the original vitamin product. 

The idea of fortifying soya comes from the notion that we get a large amount of our nutrients from cows milk and without it, must substitute. The answer in general as to 'how important' is to eat a wide variety of whole foods instead of relying on fortified foods.