I added a half price 6 bottle elderberry kit (which included some red grape juice concentrate) to a Winebuddy 30 bottle cabernet sauvignon kit, adding the flavouring pack at the outset rather than at the end. At pH 3.0, the must was overly acidic, so I added enough potassium carbonate to adjust this to 3.4. This was rather bland, so I added 3 teaspoons of grape tannin to supply a little astringency.
Fermentation is complete, but after initial racking, I have not added the stabiliser or finings, as I plan to put it into a 20 litre oak cask to clear naturally and mature before bottling. I used this technique with a Wilco CS kit, which transformed from an alcopop into a quite respectable dry red wine, although nothing like a true cabernet sauvignon.
I am only trying this experiment because the kits were half price. It would not be worthwhile at normal prices, when the next kit up (eg Cellar 7) would cost the same and produce a good wine on its own.