I have several kilos of 100% Grenadian chocolate bars that are old. I enjoyed them when they were fresh but now they are dry and, although they still 'snap', somewhat crumbly. I would like to temper them and add cocoa butter to increase the creamy taste. Is there a limit to how much I can add? Will just tempering them restore to original consistency?
Chocolate can age in different ways. The most common, and probably what happened to yours, is that it 'untempered' (blooming?). I believe simply melting in and properly tempering again will recover the brightness, texture, and flavor. If it doesn't, it might have oxidized and got rancid. This is very uncommon though since the cocoa fat is very stable and rich in antioxidants. It lasts more than two years usually.
Cocoa butter can surely be added to create a silkier and smoother texture. It is solid and stable at room temperature so in principle I don't see why there should be a limit. The science behind tempering is that of crystallization. Cocoa butter can crystallize in six different forms, each having very different physical properties (melting point, resistance to shear, etc). For this reason, it is good to keep in mind that the more butter you add, the trickier it can be to temper properly.
Personally, I find it easier to temper mixing with some already tempered chocolate, a method known as seeding. But in your case I don't know if it is worth it if this means mixing your 100% Grenadian (good stuff!) with some other commercial chocolate.