You should be able to add the custom flavor with the nova CLI, and if you need something more specific you can take a peek at this doc. Hope this helps http://docs.openstack.org/admin-guide...
Admin users can use the nova flavor- commands to customize and manage flavors. To see the available flavor-related commands, run:
nova help | grep flavor-
flavor-access-add Add flavor access for the given tenant.
flavor-access-list Print access information about the given flavor.
flavor-access-remove
Remove flavor access for the given tenant.
flavor-create Create a new flavor
flavor-delete Delete a specific flavor
flavor-key Set or unset extra_spec for a flavor.
flavor-list Print a list of available 'flavors' (sizes of
flavor-show Show details about the given flavor.