converting pure chocolate to "bittersweet" chocolate
Can you melt unsweetened chocolate, add sugar, and have bittersweet chocolate? No. The resulting chocolate might taste fine, but it would be gritty from the sugar. To get an idea of what is involved in sweetening chocolate, check out this page about making chocolate from cocoa beans: http://www.chocolatealchemy.com/conchingrefining.php

Of course, since you're using it in backing, the simple solution is just to increase the sugar in the recipe. Instead of adding the sugar to the chocolate, just add it to the cookies.