"Undrafted."

I know I have a Thunderbolts annual, special, negative-one issue; where the machinations of Zemo setting up the subterfuge of the team was explained: how he decided on the members, their disguises, and so on. I don't think the Radioactive Man was on Zemo's shortlist of potential members, partly because he wouldn't be as easily controlled as some of the other members; and partly because as he mentions, a big green radioactive man is tough to disguise.