There are two quotes in the German Kingdom Ministries that mention how many foreign congregation there are in Germany, one from 2002, the other from 2008. I made some calculations here on the German board:
http://forum.sektenausstieg.net/index.php?topic=13409.0
From the table (that is based on some assumptions on congregation sizes) you derive that the 7,110 foreign converts slightly outweigh the loss of 6,689 German publishers. Without the imigrants there was a decline of more than 5% in Germany (2001-2007)!