Add Signature in Non-Signatured Posts?

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I noticed that if you post a post/thread (etc) without having a signature and then add a signature in your user cp and make a new post/thread (etc), the signature of course is there. If you look at one of your old post/thread (etc) that was posted before you added your signature, the signature has not been updated their. If you update your signature, the post/thread (etc) with the older signature will be updated but the post/thread (etc) that was made before the signature was made, will never be updated. Is there a mod/hack/ext. (etc) to enable adding signatures to posts that never had signatures?