"Sadly" is already an adverb, meaning it describes how something is done.
If you're looking for a word that expresses a similar sentiment, you could consider:
* Sadness: This is the noun form of "sad."
* Sorrowfully: This is another adverb similar in meaning to "sadly."
* Melancholy: This is a noun or adjective describing a state of pensive sadness.
It's important to remember that English doesn't always follow strict rules with suffixes. Sometimes, a new word needs to be created entirely to express a specific shade of meaning.