If you don’t want to affect the pH you’ll all ion additions have to be in balance. I.e. if you add sulfate ions you have to add a magnesium, calcium, sodium … etc. ion as well.
You may also add ions from acid. hydrochloric acid adds chloride, sulfuric acid adds sulfate and lactic acid adds lactate. In most cases that addition of an acid neutralizes existing bicarbonate and carbonate. E.g. the added sulfate ion replaces an existing bicarbonate ion.
Unless you know what you are doing you should not mess with hydrochloric or sulfuric acid. They very aggressive. Lactic acid is much safer and, according to Narziss, results in a better flavor profile compared to mineral acids (which hydrochloric and sulfuric acids are). There might be limits though, since you may taste the lactate if you had to use a lot of lactic acids to neutralize an extremely high alkalinity (300+ ppm as CaCO2). I’m guessing here, so don’t hold me to it.
If Mg is not desired when you add sulfate you have to use gypsum. If you don’t want to do that b/c you feel that the calcium is already too high you need to lower the calcium content through dilution. This will also bring your chloride down which is desired since you are likely trying to affect the chloride/sulfate ratio anyway.
It’s important to remember that the ions have to remain in balance: you need as many (+) as (-). But don’t do that by adding up their ppm numbers. That won’t work since the individual ions have different weights. Some are heavier than others.