Looking at the MooseFS source files, I don't think they can. The license header of the files says:

   /* ...
    * MooseFS is free software; you can redistribute it and/or modify
    * it under the terms of the GNU General Public License as published by
    * the Free Software Foundation, version 2 (only).
    * ... 
    */

I think the reasoning was to add missing features, mainly HA, which moosefs did not have at the time and currently provides it in its paid version.

One thing I never understood though is why when forking they decided to rewrite everything in C++. Wouldn't be better to just write it from scratch?