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Yes, you can substitute. A tablespoon of seeds will grind down to less than a tablespoon of powder, so you'll have to adjust the amount in the recipe. If the recipe calls for a tablespoon of seeds, you will need maybe a half tablespoon of powder. Chances are a little more or less won't ruin the dish.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan