How do you convert semi-sweet chocolate to milk chocolate?

You cannot directly convert semi-sweet chocolate to milk chocolate. Semi-sweet chocolate has sugar and chocolate (without milk solids), while milk chocolate also has milk solids (milk or cream). To convert semi-sweet chocolate to milk chocolate, you would need to add milk solids (milk or cream), sugar, and possibly additional chocolate to adjust the sweetness and chocolate flavor. The proportions of these ingredients will depend on your desired milk chocolate recipe and taste preferences.