He told KryptonSite that he had attempted a number of methods to make Rosenbaum reprise his role as the villainous Lex Luthor.
"I have tried a few things with Michael," he said. "I've twisted his arm, I've taken him to dinner, I've gotten him drunk. I'm trying everything that I can."
Welling confirmed that Rosenbaum is interested in making an appearance in the final season of the programme, but that no deal has yet been made.
"Michael wants to come back, but there are just some things to iron out, and I think he understands that we can't end the show without Lex Luthor," said the star.
He added: "We're even teasing the idea that he's going to be on already, with the idea of clones, or [the idea] that Lex has developed these bodies that he could then tap into to heal himself later in life."
Smallville will return for its tenth and final season on September 24 on The CW.