For what it's worth, here's a small workaround I discovered today.
It seems that Shopsite and Google Webmaster Tools are out of synch.
The Google Sitemap wizard in Shopsite only wants me to verify my site with Google using the html upload method. But that method doesn't seem to exist in the Google Webmaster tools menu. It wants me to add a meta tag to the header of my home page.
I did that easily enough by editing my template.
But when I wanted to add a sitemap, the Shopsite wizard still insisted that I use the html upload feature.
I simply gave the wizard a silly file name, clicked next, and went on to the sitemap section.