Perhaps this can be helpful?
Someone at the Danish National Web Archive  (http://netarchive.dk) has made a
plugin for HTTrack that makes it archive in ARC format. I havn't tried it out,
but that is what it claims to do.  

<http://shasta.cs.uiuc.edu/~lrclause/arc.c>


Kind regards,
Bo Hovgaard Thomasen