Hi,
It will require a custom plugin to perform such kind of task.
My advice would be to integrate with a component made for send newsletter (so I think about Acymailing) because if you have a lot of users, it can be a lot of emails to send.
The idea is to catch the event when a product is approved (onAfterProductUpdate) and then create a Acymailing newsletter.
Otherwise you can see to send directly the emails but depending the quantity, you will probably timeout and the approval in the backend (or in the frontend) will need a lot of time !
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.