I jokingly asked Isaac Sopoaga this week about a position switch to fullback.

In response, the 49ers defensive end, clearly thrilled by the suggestion, leaned in closer to my tape recorder and delivered a message to his head coach.

“Oh, that would be nice,” Sopoaga said “Please, Harbaugh, fullback.”

The 320-pound Sopoaga, of course, isn’t moving over to offense – his two-play stint at fullback in last year’s win against the Raiders notwithstanding.

But a position switch from defensive end to nose tackle isn’t out of the question in 2011 if the 49ers can’t re-sign Aubrayo Franklin, an unrestricted free agent.

If Franklin leaves, it’s conceivable the Niners could fill the hole in the middle with Sopoaga, a seven-year veteran who spent his first four seasons at nose tackle. Sopoaga, 29, said the Niners haven’t discussed a potential switch with him. Jim Harbaugh was the only coach he met prior to the lockout and he isn’t even sure what defensive coordinator Vic Fangio looks like.

But Sopoaga understands such a move is possible. Last year, Franklin, looking ahead, said he was eager to test the free-agent market. It remains to be seen if backup Ricky Jean-Francois, a seventh-round pick in 2009, is ready for full-time duty if Franklin exits.

“To be honest, I would love for Aubrayo Franklin to be back with us,” Sopoaga said. “But whatever comes through, I don’t mind doing it because I did it my first four years … I’m a happy guy. It doesn’t matter where they put me. I say I’m ‘The Bullet.’ Just load me in and just go. I’m comfortable playing nose tackle or D-end. As long as it’s on the D-line area, I’m fine.”

Sopoaga was listed at 330 pounds last year, but he said he played at his current weight of 320. A former rugby player in American Somoa and one of the strongest players on the team, Sopoaga doesn’t plan to bulk up if he moves back to the middle.

“To me, 320 as a nose tackle is heavy enough,” Sopoaga said. “I’d rather be comfortable than being just a big guy. I want to chase plays, beat the guy in front of me before the quarterback can hand off the ball to the running back … I used to be that (big) guy my first four years when I played at 335 or 330. It was OK, but it was not fun. It’s fun making plays on the line of scrimmage, but it’s a greater feeling when you make plays in the backfield. That’s my thing.”

• I asked Sopoaga about linebacker Aldon Smith, the 49ers’ first-round pick who began conditioning with a group of veteran defensive players last week. Sopoaga said the 21-year-old Smith’s introduction to the NFL was a little, um, messy.

“I wish his agent would have warned him about this being a different level of training,” Sopoaga said. “I was kind of feeling bad for him when our first time last week he was running the hills and he was drooling, he was throwing up. I was thinking ‘Uh oh, come on now. Welcome to the pros.’”

Sopoaga said Smith ran the bleachers Monday at Spartan Stadium, but didn’t work out Tuesday because of tightness in his back.

“He said he needed a back massage because his back was tight,” Sopoaga said. “It was funny … But he’s a good kid. He’s a good guy. I believe Aldon Smith will make plays.”