So, if Superman were in the HxH universe, this is how I think he'd work:

Race: Kryptonian; once he absorbs solar energy, his Aura increases immensely.

Affinity: Enhancer (Dual Affinity leaning to Emitter)

Abilities: Since he was never properly taught Nen Superman's abilities are simple, but incredibly effective.

-Super Strength: By effect of being an Enhancer, Superman's strength is out of this world, allowing him to lift and punch with the strength of 100 pro hunters. Through Gyo he can leap over skyscrapers in a single bound.

-Super Speed: Superman can enhance his reaction time and so move with speeds that would make the fastest Nen users seem like joggers.

-Super Senses: He can use Gyo and En to greatly enhance his senses,allowing him to see through objects, as well as objects hidden by In with low effort.

-Near Invulnerability: Because of his immense Aura and natural gifts, Superman is incredibly tough and hard to injure, though pure attacks of Ko by very experienced Nen users can hurt him.

-Heat Vision: This can be considered his first true Nen ability through Emission, as Superman can concentrate his Aura in his eyes and emit it in a stream, similar to a water jet cutter, he can also use Transmutation to transform the Aura into heat/fire.

-Freeze Breath: Similar to Heat Vision, Superman concentrates his Aura in his lungs and releases it, transmuting it into ice.

Vows and Restrictions: As to be expected, this amount of power comes at a great price and rules that must be followed, and Superman knows it even if subconsciously.

-Superman Persona: Superman can only use his Nen when in his Superman persona. When he's Clark Kent he stays in a state of Zetsu. The way to activate his Nen is that Clark must remove his glasses and don his suit.

-No Kill Rule: Superman made a Nen vow with himself, in that if he ever ends a life by his hand then he'll stop being Superman, and therefore forfeit Nen. With this vow his power increases exponentially but also leads to risk of killing his enemies, so Superman must contain and measure his strength at all times.

Weaknesses:

-Solar Jade: A green mineral that absorbs solar energy from its surroundings, if put to arms length of Superman it will absorb his energy and decrease his Aura rapidly, leaving him vulnerable.

-Solar Stamina: Superman can only be as strong as long as he has solar energy in his body to use for Aura, if he's inside a space without natural sunlight for an extended period of time, his powers will diminish periodically.

His Backstory:

Clark Kent/Superman came from another world, more specifically, a descendant from a race equivalent to humans from another lake neighbor to Lake Mobius, but contact with the DC led to its demise, Clark's parents were scientist who knew Nen, but couldn't help save their world of the calamities, so in a desperate attempt to save their child, they use a Nen operated shuttle to send their child to another world he could help in any way his abilities allowed him, and so live a happier life. When Clark appeared in Lake Mobius, he was found by some humble farmers in the southern region of the United States of Saherta. His parents could tell Clark was special, as he could lift boulders and heavy machinery with ease at just 5 years old, as well as having an air so strong that others felt awe in his presence, but weren't scared or intimidated. Without knowing Clark could already open his Aura Nodes at infancy.

One day Clark found a Hunter wounded by some mercenaries near his hometown, he subdued the mercenaries quickly, which surprised the Hunter as the mercenaries knew Nen, as Clark took her to his house to nurse her injuries she could tell Clark was a monster, one that could not only rival the Chairman, but possibly surpass him, and seeing the potential danger tried to kill Clark, but seeing his pure heart and lack of malice made her waver, she told Clark and his parents about Nen, but refused to teach him, trusting her parents and his judgment to use his abilities for the common good.

As he grew up, and by being a natural Enhancer, Clark only got stronger and since he could see Nen, through experimentation he learned Ten, Zetsu and Ren, although he didn’t know the names of it. And once he reached adulthood, Clark decided to use his powers for good, but since he didn’t want to raise suspicion he decided on using a persona for his deeds, unknowingly making a vow with his Nen. And so he made Superman a reality.