While you've listed off three questions about adding hydration, the first question you should be asking is:
Is my cat dehydrated?
It's very easy to check this yourself at home by using a pinch test. As described by PetMD:
If you take a pinch of skin over the cat's shoulders and pull up gently, the skin should snap back into place when released. As the cat gets more dehydrated, the skin goes back in place more and more slowly. If the pinch of skin stays up (the "tent"), it is a sign of severe dehydration. The cat should be seen by a veterinarian immediately.
Should a cat receive hydration from only wet food?
No. While cats are designed to get most of their water from food, you should always provide clean, fresh water for them as well; you may find a benefit to trying different types of bowls or fountains to increase the appeal.
Is there a benefit to adding water to wet food?
Yes, if your cat regularly suffers issues with dehydration, adding water to their wet food can help keep them more hydrated. I've seen this used (on veterinarians' orders) with a cat with chronic urinary crystals to help flush them out before they became large enough to cause problems. For most cats, there won't be much need to do this, however. Your vet can offer more input on your specific cat's case.
How to add the water?
Just mix it into the canned food. This tends to work best with "pate" style foods, making them into a thinner mush or even a "soup," and not as well with shredded or "chunk" style foods. There's no need for any special "gravies," which provide very little benefit for their price, and may very likely contain extra ingredients that are not ideal for cats to eat.
Will it spoil faster?
No faster than wet food on its own will. They should be eating it promptly; you should not be leaving it out for long periods, with or without added water.
Is it necessary?
If your cats are already getting sufficient hydration, then it's not going to provide much if any benefit to add even more water to their diets. Just because you aren't seeing them drink water, that doesn't mean they aren't drinking it. Start by checking whether they're at all dehydrated, and consult with your veterinarian if you have concerns about their hydration levels.