Koschnicke will join the WPP firm on 1 January as a partner.
He will join a corporate and public affairs team led by partners Henriette Peucker and Christoph Bertram, which advises clients from its Berlin and Brussels offices.
In April, Koschnicke joined the HQ of Merkel's CDU party ahead of a federal election in September at which the party retained its position as the country's biggest party, but lost ground to rivals.
Koschnicke has also worked for the CDU in two previous elections, and been on its staff for more than a decade previously, before moving into the private sector with German polling firm Forsa. He then joined vehicle manufacturer Opel, where he was vice president of public policy and industry strategy between 2013 and March 2017.
Ralf Hering, principal partner of Hering Schuppener, said: "Putting complex transactions and crisis situations into a societal and political context is an intrinsic part of our integrated consulting approach and strategic corporate communications.
"Joachim Koschnicke, with his deep insight into German politics and his comprehensive campaign experience for internationally operating companies, will therefore be an asset for our team and our clients."
Hering Schuppener announced its alliance with fellow WPP firm Finsbury in April 2016, and US firm Glover Park joined this alliance in May this year. The alliance has a total of 500 staff; 100 of these at Hering Schuppener.