You can. All you need to do is put a statement into the released version of the text saying that you have.
If you want to release it into the public domain and distribute it through Project Gutenberg, you should send us a letter to that effect.
To: Michael S. Hart
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Dear Project Gutenberg:
I am the sole copyright holder for the book, "Wallaby Happiness." It
gives me pleasure to release this work into the public domain, and I
invite Project Gutenberg to publish this public domain edition.
Sincerely,
Gregory B. Newby
Once you have released it into the public domain, neither we nor anyone else needs your permission to publish it, but for us to be sure that it is a public domain version, we do need a signed letter.