Extract electric field at a surface in 3D model and then re-import it back as a new exciting source

I am trying to extract the electric field of a surface in a 3D model and reuse it as a source to excite new fields. The problem is we can extract the field of a 2D surface, but cannot import the extracted data back as a source, since we do not know how the coordinates at the surface and in the 3D model are linked. Does anyone has any ideas how to extract the field and put it back at the same surface as a source?

Thank you, Jie