Check out the threads “Making an FPAC” and “Tweaking Cheap Kits” (pretty sure those are the thread titles).
Joe (@joeswine) has mastered the art of improving kit wines, including the addition of fruit.
To your question:
I would add the fruit in the primary after cooking it down with some sugar and compatible wine (cook off some of the liquid and make it mushy). It will add body, flavor and bump your ABV a bit.
If you’re going to add fruit, you’ll also need to add pectic enzyme to help your wine clear.