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You're better off with 6 x 6 treated posts if you stay with the same number. If the porch has been stable all these years then adding more posts isn't critical. You can add more posts but you'll also have to pour concrete piers that extend below the frost line to support each post.
You don't need plastic and "plastic" isn't structural. PT will probably last longer than the original posts.
I'm assuming you mean the posts that support the deck.