Koeppel has been tasked with growing US client base and strengthening its successful New York team.
She joins from The MSL Group, the New York-based creative and production company where she was managing partner and executive creative director.
Prior to this Koeppel founded ATM Design where she pioneered the concept of 720 Degree Brand Integration. ATM Design was acquired by MSL in 2004.
In April the company hired John Mathers, former chief executive officer of the Brand Union, as chief operating officer to grow its international business.
Mathers said: "Both Stef and Jean have experience of dealing with international senior-level clients and have worked on some of the world's biggest global brands.
"This will be vital for Blue Marlin as we move on to the next stage of our development, and I look forward to working with them as the company evolves."
Blue Marlin has an international network of offices across the UK, US, Australia and the Far East.
Clients include Cadbury Schweppes, Shell, GSK and Reckitt Benckiser.