These processes are quite a bit different after the FD step. In the case of E6 you have a color developer which forms a dye wherever silver halide is present, after which a bleach&fix step removes all silver. In the case of b&w reversal you remove all silver, then develop the remaining silver halide, and the thereby created silver is not removed but actually is the final image.
Therefore I don't think you can reuse much between E6 and b&w reversal except, maybe, the FD. You could, of course, also use the E6 CD as b&w redeveloper, but there should be cheaper alternatives.