The only way that I know of to "thicken" a thin wine is to barrel age it. For a pieseporter, you would need an extremely neutral barrel to keep it in character. Barrel aging concentrates a wine due to micro-oxygenation and evaporation (of course you have to top up).
Appleman is right about the body in most kits. The higher the concentration of juice to concentrate makes for fuller bodied kits. Reds with grape packs (Crushendo, Winery Series, and CC Showcase) tend to have the most body without modification.
Now, with that being said, note that modifying your kit will VOID THE WARRANTY. Whites can benefit from golden raisins added to the kit to raise the body and mouthfeel of the wine. Bananas will help with both white and red kits. Bananas do not add flavour, so don't worry about that. Typical doses are 1-2lbs of frozen, black bananas per 6 gallons. Get the bananas into a mush and just add straight to the primary.
Chemically, what the bananas add is beta-glucan which is an unfermentable sugar. It is the same stuff that makes oatmeal sticky. It is an important ingredient in both wines and beers to make the wine "thicker"