Here's why and how you can express the idea:
* Smiling is a verb, so it describes an action. Adverbs describe verbs, adjectives, or other adverbs.
* To express smiling in an adverbial way, you'd need to use phrases:
* "Smilingly" (although this isn't a standard word, it's used in some contexts)
* "With a smile"
* "Smilingly"
Example:
* Incorrect: He walked smiled into the room.
* Correct: He walked into the room with a smile.
* Correct: He walked into the room smiling.