compute a pump and merge meshes in OpenFOAM
 

I also have the problem which i want to compute is a pump contains three domains: inlet domain,impeller domain,volute domain. they are meshed individually.
Now i do not know how merge the three in OpenFOAM, and how can i set the initial conditions in directory 0/U and 0/p and so on.
Anyone can help or deliver the U 、 P and others ?