Re-corking wine and Making it last

It sounds as if you want to make a wine punch and rebottle it. If you add a huge amount of a liquor containing a lot of alcohol - say wine and strong brandy mixed half and half, that will prevent the mixture from going bad. The taste of added fruit juices may change in a rather nasty direction due to enzyme action. It might require rather severe pasteurization or microfiltration under sterile conditions to prevent this, and the fruit might have a cooked taste if pasteurization is used. Neither process can be done at home in an inexpensive and safe manner.

If you do not add a large amount of alcohol, watch out. The added sugar and sugars in fruit juice are likely to ferment when sealed in the bottle, because yeast cells are nearly everywhere. If you use just the right small amount of sugar, you will get a sparkling wine and some sediment. If you use just a bit too much sugar, either your bottles will explode, or the corks will be shot out. At worst you could be cut by glass, and at best you will have a huge mess to clean up.

No, the wine will slowly oxidize, with reds taking on prune-like (or Port-like) flavors and whites tasting like Sherry or browned apple.

The problem with what you're proposing arises from the fruit juice and sugar, not the wine. Your mixture will start to referment after a few days due to the bacteria that'll inevitably sneak into the bottle when you open it. A secondary fermentation produces carbon dioxide, which'll create pressure inside the bottle and blow the cork out. It'll also make your mixture taste sour with time. If you add enough liquor, you might be able to prevent that from occuring by having enough alcohol in there to kill any bacteria. If you start with Port, that's pretty much a cinch, though why you'd want to add sugar or liquor to Port is a mystery to me. If in doubt, you could always Pasteurize your mixture to keep it from a secondary fermentation.

Have fun, Mark Lipton