I'm not sure what you mean. I take it you have a kernel in your RiscOS
partition which you have managed to start RiscBSD from. That file just
has to be copied to /netbsd in the NetBSD side. No extraction is
necessary. If you didn't manage to to start RiscBSD with that file,
then it may well be compressed, in which case "gzip" is the utility
you want to uncompress it. Or is it the copying from RiscOS to RiscBSD
which is the problem?

Patrick