Hi,
Technicaly, You may simply add an event to the associated activity notification.
Then you can manage the condition (may you want to verify any prior to send your notification) and use a multiple notification macro as the action to send out.
That say,
I don't think this is a good approach if you just notify the helpdesk but the ticket remain on the support group.as there is no accountability of the Service Desk
My approach will be using similar technic of above to transfer the ticket back to the Helpdesk.
Then the helpdesk have a better visibility (as ticket is in their queue) and is accountable to reachback to end users. You also better measure OLA of each and others.
My 2 cents.
8/J