Um. Sure. A Spartan-III that died in August 2552 is the same person as an ODST who was active in New Mombasa in October 2552 and later died in January 2554. Because that somehow makes any sense whatsoever.
The Spartan III decides to abandon the Mjolnir that very few III's ever got, somehow gets off the planet to become an ODST and get stationed on a Cruiser in Sol in a month then a couple years later on a mission forgets he's superhuman, gets captured,by rebels, has the crap beaten then gets executed.
Canon-wise, they are already 2 separate characters. But consider this: should they be a single character, 2 negatives will not make a positive and the character would simply be double-dead. Let the dead rest.
oh boy, he got to die twice apparently. seriously though, for what reason would 6 even become an odst? I don't know about you but I'd like to keep the armor with built in shields, and I know many spartans are on the same page. this is also all ignoring the fact that we watched him die.