Can I make milk hot chocolate out of dark chocolate and sugar?
Your premise appears to be that the difference between milk chocolate and dark chocolate is merely milk and sugar content, such that if one blends all three together, in any order or manner, the results would be 'the same' (or at least, taste the same). This is not the case. Not only would the ultimate ratios of cocoa-sugar-milk end up being 'off' dark chocolate contains ingredients that are not used in milk chocolate, lecithin (from soy or milk) to emulsify the chocolate liquer and sweetener in order to create a smooth product. This is a different process than simply melting solid chocolate into milk.

While you could ultimately end up with two similar products they would not taste 'the same'...at least to a discerning palate. 

For some additional information see: What is the difference between Milk Chocolate and Dark Chocolate?  and Milk vs. Dark vs. Semi-sweet vs. Bitter-sweet chocolate?