Just for clarity's sake, do you want to actually overwrite the iWeb site (as in delete it) or just use Rapid Weaver to edit your iWeb site on .Mac?
I have had no success in Rapid Weaver with editing iWeb sites and have had limited success using Dreamweaver (often the layout gets really screwy when I try to edit an iWeb site in Dreamweaver). Any one else have experience editing iWeb sites with Rapid Weaver?