Filling tha GAP between two meshes
 

Hello everyone,
I'm facing a problem:
1) I created a 3D mesh with snappy tool.
2) I made it a 'fake' 2D mesh (aka 1 cell along z direction)
3) Now I made a mirror of this last mesh
Now my problem arises: since there is a empty space between this two block (original and mirror), is there any tool in openFoam which can fill this gap? :confused:

http://postimg.org/image/iewzifqq7/

Now the faces adjacent to the gap have same mesh seeding. My question is how do I fill this gap by extruding these faces creating 'n' layers in between (aka meshing the gap)?