Shalom, if a Jew bought a grape juice that is not kosher, and then added water to that juice, would that non-kosher juice become shehakol, and can a Jew drink it by making brachos of Shechakol?
It is not clear from your question why the grape juice is not kosher. But on the assumption that it is stam yanom, adding water to it will not help unless the mixture is 60 parts water to the grape juice, therefore we may not drink it. The bracha might technically be shehakol, however the rule is that food that is prohibited to eat does not get a bracha, (unless done for medical reasons etc.) because it is not praising Hashem, rather angering Him by making a bracha while doing something forbidden.
Shulchan Aruch Y:D 134-2, O:CH 196-1.