While an alkali addition might correct an overly acidic beer, you need to be sure that the beer is overly acidic in the first place. If this was a stout made with very low alkalinity water such as distilled, RO, or rain water, then its likely that its overly acidic. In that case, the alkali additions mentioned above could be prudent. I would conduct tests in a glass of beer first, before adding any minerals to the keg. Once you add them, minerals are impossible to remove. So be sure that you the fix is going to be correct before committing the batch.