Can I marinate already cooked chicken?

Online Answer
As for flavor, long marinating usually doesn't add much more flavor than short marinating. It is better, easier and faster to brush food with liquid during or after cooking. And for the strongest flavor, marinating after cooking is the way to go. This technique, usually called escabeche, was once used to preserve food.
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Hard Vegetables (Potatoes, Carrots, Beets, etc.): Marinate these veggies for at least 30 minutes and up to overnight. ... Don't marinate these for longer than 30 minutes because they will release water and get soggy.
The salt in your marinade is important, but overly salty meat can lead to a dry and tough finished product. Keep this in mind when you're preparing your marinade and don't go crazy with the salt, because it pulls the moisture out of your ingredients.
A great marinade is carefully balanced and made of three basic components – acid, fat and seasoning. Acids, such as wine, vinegars, citrus juice, buttermilk and enzyme-rich fruits like papaya or pineapple, work to soften the meat's surface by weakening proteins allowing for slight absorption of flavoring..
According to foodsafety.gov, it says you shouldn't have chicken in a marinade for longer than two days in the fridge..
But with tougher cuts of meat like skirt steak, a marinade can make a big difference. Flavorwise, you don't usually need an overnight marinade, though it certainly won't hurt. The truth is, the marinade isn't really absorbing into the meat past the top layer, so even an hour is usually enough to get some nice flavor.