You would probably have to personify the thoughts after the thought and still imagine him saying it. And like translate the diction to have him say it about you even if your thoughts to say I.

"My good friend, one of the smartest men on my team, RFJ jr, he said ADHD is bad. It only makes life worse, he said. No one should have it. No one. That's what he said. I have to agree. Have you seen (insert your name), folks? (Your name's) life would be way, way easier without it. I can tell you that. It's so sad."

That's my shit attempt at an impersonation.

Not all activities are for everyone. It's possible this isn't for you, but it could be feasible if you're still able to imagine him or someone else you dislike saying it after.

I felt this sort of thing was extremely useful for my eating disorder recovery though I had a different voice. I should probably try it with other negative thoughts. It does take a certain level of being able to ground and slow down and catch your thoughts as they happen or being able to reflect on them later.

Some people think YOU to themselves instead of "I". "you" thoughts are easier to make less ego syntonic and to personify as something else for sure though. I think "I" thoughts a lot too and just had to translate them over into "you" thoughts to imagine a bad person saying them.