Exhibit Builder doesn't currently have a feature like this.
It's a good idea, and several people have suggested or asked for it before. It's among the (many) things we're considering for the Exhibit Builder in Omeka 2.0.
For the time being, your best bet would be to write a plugin that reads the Exhibit data and creates the XML or RSS output you're looking for.
The title, description, and slug of an exhibit are all directly accessible from the Exhibit object or the exhibits table, and the tags are accessible from the Exhibit object as well (though they're stored in a different table).