Is it possible to sweeten chocolate without making it gritty?

Doing both is possible, but there is no point. For both, you need a conche, and this is a good contender for a gadget not worth its shelf space in a home kitchen. For a good chocolate, you have to mix the melted combination of base chocolate and additional ingredients for hours (Wikipedia says 6 for low quality, up to 78 for best quality), and this is what the conche does for you. For a nice structure and shine, you also want to temper the chocolate afterwards and cool it under appropriate conditions.

Chocolate is made by pressing roasted cocoa beans to cocoa liquor (consists of cocoa butter and cocoa solids). The dry remainder of the beans is then ground to cocoa powder (and sometimes also deacidified in a Dutching process). The cocoa butter can be separated from the liquor. The pure liquor can be tempered and sold as high-end baking chocolate, also called "unsweetened chocolate" in the US (low-end blocks of "baking chocolate" are mostly cocoa powder mixed with hydrogenated vegetable fat). To create bars of chocolate, chocolate creators like Lindt put cocoa liquor together with sugar in a conche to create different grades of dark chocolate (US name: semi-sweetened chocolate), or also add milk or milk solids to create milk chocolate. Lower grade semisweet chocolate has cocoa powder added, so a 72% chocolate will always have 72% cocoa bean products and 28% sugar, but depending on the brand, the ratio of cocoa solids and cocoa butter will vary, which will have effect on its behavior in confectionery (a higher ratio of cocoa butter will produce a smoother chocolate with more shine).

If you insist on changing the darkness of a chocolate you have and achieve reasonable quality, you have to throw together the needed ingredients, conche them, and temper them. For example, you could start with 100 g 60% semi-sweet dryish chocolate, add 80 g cocoa butter and 20 g cocoa powder, and you will end up with 200 g of smooth 80% semi-sweet chocolate, if you do it right. Given the prices for high quality raw materials, energy requirements and space taken up by a conche, and the availability of different grades of chocolate, I don't see any reason to go through all of this instead of directly buying good quality 80% chocolate.

This all assumes that the final product you want is pure chocolate. If you are making, say, a chocolate flavored custard, where you mix ganache into custard, and you have a chocolate bar darker than what the recipe specifies, you can get a perfectly good solution by first dissolving sugar in the warm cream and making the ganache with it, and then adding it to the custard. Other recipes which work with chocolate mixes (e.g. cake layer batters) function in a similar way - add your ingredient (cocoa powder if your chocolate is too light, sugar if it is too dark) to a wet ingredient which will dissolve it, and then mix the melted chocolate in as usual.

The only practical way to do this at home would be to combine two chocolates to end up with something in the middle. Say you have some extra dark (80% cocoa solids) chocolate and some semisweet (60% cocoa solids), you could melt and combine them to make anything from 60% to 80%, depending on the amount of each. In other words, if you want to sweeten chocolate, do so by mixing it with other chocolate that's sweeter than what you're looking for. That way, you're essentially adding sugar, but it's sugar that's already been mixed with chocolate and put through the conching process.

I could see doing this if you were planning on melting the chocolate anyway, perhaps to coat something. In that case, it's really no big deal -- melting two kinds of chocolate is as easy as melting one kind. Otherwise, unless you live far from civilization or your last name is Torres or Sharffenberger, it's probably faster, easier, and more reliable to make a trip to the store to pick up the variety that you want.