Phaneuf had 16 points (three goals, 13 assists) in 53 games with the Senators this season.
The Kings ran line rushes during their hourlong practice but did not show set defense pairs. Phaneuf spent most drills with defenseman Alec Martinez.
Kings coach John Stevens said he is unsure where Phaneuf will be used against the Penguins.
"It gives you a veteran guy on the back end that can play both sides and play in all situations," Stevens said. "Gives you a lot of options. I'm not sure exactly how it all lays out just yet, but for sure, gives us a lot more depth on the back end."
No matter where Phaneuf plays, the 32-year-old said he does not plan to change his style.
"Where I fit in, as a role, that's up to the coaching staff," Phaneuf said. "But for me, as a player, I want to bring and do what I do. I've played a physical style of game for my entire career and that's something that I'll have to bring here. You're playing in the [Western Conference]. You're playing against some big hockey clubs. For me, I'm going to come and bring that edge to my game, bring the edge with my game, and try to contribute in any way that I can."
Stevens said he expects Thompson to make his Kings debut Thursday. Los Angeles was told Thompson, who has missed four games with a lower-body injury, was set to return for Ottawa on Thursday, Stevens said.
The Kings planned to check Thompson when he arrived in Pittsburgh later Wednesday.
Defenseman Drew Doughty said he expects Phaneuf and Thompson to become locker-room leaders.
"I think [Phaneuf] is going to really help our team, and so is Thompson too," Doughty said. "I think that's something that we really needed in our lineup, was some more veteran presence."