Once a package is mounted it's always mounted.
As Don said, if you're trying to retrofit existing bulk-mail instances with attachments, an *-init.tcl file that searches the site-map for existing bulk-mail (without attachments child node) and instantiates attachments would work. You should only have to do this once to hit every existing bulk-mail node.
If you add code to the bulk-mail after-mount callback to instantiate attachments, each new (mounted) bulk-mail instance would automatically get attachments.
Procs that may be useful:
site_node::get_children
site_node::get_from_node_id
site_node::instantiate_and_mount
