Gary and ECCE users,
I couldn't find an officially recommended way of adding basis sets to ECCE, and so ended up writing my own set of scripts to do so.
In case anyone else is interested in converting nwchem basis sets to the ECCE format, I've put the scripts at http://sourceforge.net/projects/nwbas2ecce/, together with ECCE basis set files for def2-svp, def2-svpd, def2-tzvp etc.