I am wondering if it is possible to modify a single graph object by multiple threads which attach edges into seperated regions of the graph? To be exact: I have a set of vertices in my graph with index 0-maxIndex which I want to copy a couple of times into that same graph structure one after another. Of course, I assure that there are no resize events during copying and no overlapping. This should be possible, I think, at least in theory, but I found no word about that in the docs of the BGL. So, should it work or not?

It depends on your choice of graph data structure. If you're using an adjacency_matrix, then trivially yes. If you're using adjacency_list, then you may have problems. The adjacency_list contains a global list of edges that is modified on add_edge so you're probably going to run into race conditions.