adding additional regions into BEM forward problem is as simple as adding more fields to the BuildBEMatrix module and setting the appropriate  parameters.  For instance, adding lungs to the example would involve loading a lung file, setting it as one of the inputs to BuildBEMatrix, then setting the field parameters in the module UI to measurement and the inside and outside conductivities.  You should try to make sure that the outside conductivity of the lung matches the inside conductivity of the torso.  Another detail to consider is to make sure that the surface normals are correctly oriented, as this may affect the solution.  You can use flipSurfaceNormals to flip them.  The output of the BuildBEM in this case would be a matrix of size all source nodes (heart) and all measurement nodes (lung and torso).  You can use SelectSubMatrix to extract the portions of the matrix that relate to the respective surfaces.  

Another option for an inhomogenous forward problem is to use FEM.  There are examples and a description in the forward inverse toolkit.  

Cheers,

Jess