How to turn pasta sauce into a longer-lasting ketchup? I was making a bacon sandwich earlier and I have some nice jars of tomato pasta sauce in the cupboard. I thought it'd make a great relish but it would go off too quickly in the fridge to make i economical. It got me wondering whether there's anything you can do to make an existing tomato sauce into a something with a longer shelf life.

For example, could you add vinegar or salt to tip the sauce over into more of a preserved sauce? 

Similar question might apply to salsas, I suppose?
Ketchup keeps because it is full of sugar (reduces water activity) and vinegar (acid, keeps microbes and especially molds at bay). A boatload of both. Look at a ketchup recipe for reference...