Is it possible to 'denormailze' an audio file?

Not really. The process that does that isn't so much normalization as it is compression. An Expander (e.g. Cakewalk's Expander/Gate Prochannel module) can add dynamic range, but it won't restore the original dynamic pattern, and it's not intended to be used on a finished mix. There might be some third-party plugins out there that work in a more sophisticated way, but I wouldn't expect them to be both cheap and deliver really amazing results.

The process of compression is a destructive one so it's not reversible in the sense of being able to recreate the original material. Many different sounds could, with different degrees of compression, result in to the same output. This means trying to work backwards from the output to the input is impossible as you don't know which of the possible inputs it was.