Nwosu becomes a partner, and will team up with Tim Allnutt, the former managing partner at CHI's Naked Inside joint venture. The pair worked together at Lowe London.
They will restructure the CHI planning department to merge communications and account planning into a single discipline. Naked Inside was folded into the main agency last month.
Allnutt said: "Today, there's no place for silo thinking, so our task is to bring communications planning even closer to the account planning discipline. We call this new approach 'ideas planning'."
Nwosu added: "I'm joining at the best possible time. Everyone here is talking about the next five years, no-one's looking back. Teaming up with Tim again and being made a partner was just irresistible."
Nwosu and Allnutt will also work with WPP's Group M, the media agency that will support CHI's full-service media planning and buying offering to clients. Last month, CHI sold a 49 per cent stake to WPP for an estimated £30 million.