Can you season skewers after cooking them?

It's very common to season steaks or other meats with salt and pepper after cooking, there would be nothing unusual about that. There are also seasoned salt mixes which people may use as well, that's also common. Using other spices is less common, there are finishing spice mixes which could be sprinkled on, which is less common.

As a writer myself I would say it will work any way you want it to. I would think nothing of seeing someone sprinkle some salt on food after cooking, unless I had reasons to be suspicious to begin with, and then I would touch and smell. Or I might not have suspicions until I smell the food and detect something odd.

There are a few things that I can think that you might legitimately add after you've cooked the skewers:

• fresh herbs, chopped fine (for color / decoration (aka. 'garnish') & to add a "fresh" quality)
• spices that would burn over high heat (black pepper being one of them)
• sauces, to add moisture. (especially those with sugars that might burn when cooking ... although you'd likely marinate the meat in it, so it get charred, then add more layers of it as you cook, then top with fresh stuff when you're done)
• citrus juice, to add some acidity and balance to an otherwise fatty dish

It's also worth noting that although some spices are enhanced by heat (like "blooming" spices in hot oil), putting them on at the end, even if they're not heated, can help to enhance our perception of those spices, as they'll be more likely to contact your tongue than something that's inside a mixture. (in the case of kebabs, between the chunks of meat)

Sometimes you forget an ingredient when cooking something. If the ingredient is a spice, herb or flavor extract, you might sprinkle a bit of it on after the dish is fully cooked.

The character could also say, "Oops, I forgot to add the ___. I'll just sprinkle it on now, and you won't even notice the difference."

It's especially plausible if they glance at the recipe right before saying that.

The character is not good at cooking. He made all kind of obvious mistakes, like not pating the meat before searing it, burning the garlic, not drying the lettuce. So, anything weird will be attribute to his inexperience.