According to this bittersweet chocolate definition, what's sold as "bittersweet" can range from 35-70%. Accordingly, you will need to add some sugar to have the recipe turn out as planned. My guess is that bittersweet "chips" are on the sweeter end of this rather large scale of what qualifies as "bittersweet" chocolate, but you can probably go to anything in that range that is to your taste. I would probably try something like 50% (it's in the middle and it's an easy ratio to measure) and see if you like the flavour of the final recipe.
I would probably add the sugar to the recipe itself, rather than trying to modify the chocolate since melting the chocolate will affect how it behaves once it's re-hardened (see discussions of tempering for details).
Normally, bittersweet chocolate would be composed of the unsweetened chocolate liquor, cocoa butter, and sugar. I'm not certain of the role of the cocoa butter in incorporating the sugar and whether or not it would be necessary to add or substitute for that.