Hello,
A plugin can't be override.
You can customized the display using CSS but if you want to change more ; you have to duplicate the plugin and have your own.
The goal of the "add to cart notification" plugin is to provide a basic feature and also to be an example for new cart notification plugins.
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.