1) If you have some sort of refiner/conche setup you could melt the unsweetened chocolate, add sugar, and grind away until it was smooth. How much sugar to add would be something you'd have to experiment with. (You can also pre-refine the sugar to reduce the particle size before you begin - a food processor works for this.) Whatever you do, do NOT add regular confectioner's sugar which has cornstarch added to it to prevent it from caking. You will ruin the chocolate that way.
2) Melt the unsweetened chocolate and add melted semi-sweet chocolate to it. Again, the amount of semi-sweet you'd add depends on the level of sweetness of the semi-sweet chocolate and how much you want to sweeten the unsweetened chocolate.
Also, depending on what chocolates you're using, you might need to add some cocoa butter (or alternatively lecithin) to thin the chocolate out to the correct working consistency.