You surely could say that colloquially but I am not sure if it is standard, one has to consult a grammar handbook. And I am struggling locating the relevant entry in a handbook.
In the Czech national corpus all "Němcovi" I can find correspond to men who have their surname "Němec", not to actual Germans. If it were Mr. Němec (a common Czech surname :) you could say "o panu Němcovi" for sure. I think you will find exceptions, I am sure you can say what you did colloquially, but it may be non-standard.