Addo transferred to Borussia Dortmund in 1999 and played more than 75 times for the team, becoming Bundesliga champion in 2002. However, he was also hampered by great injury troubles, as he tore his cruciate ligaments three times in this time, the first coming after a German Cup match against SC Freiburg on 15 July 2001. The player had an MRI scan which showed he had torn knee ligaments and underwent surgery on his right knee on 22 July at the Vail, Colorado clinic of world-renowned knee specialist surgeon Dr. Richard Steadman. He healed completely after that, and came back to play the 2002 UEFA Cup Final for Dortmund against Feyenoord Rotterdam on 8 May 2002, which Dortmund lost 3–2 at the Feijenoord Stadion in Rotterdam. On 7 September 2002, Addo tore right knee ligaments for the second time, in a 2004 African Cup of Nations qualifier versus Uganda in Kampala.