Hi,
It is currently not possible unless you modify the HikaShop core.
We planned to rewrite the "add to cart" system and add some javascript events (triggers) in order to add new features and also let you (and the other users) implement your own javascript code using the events.
But for the moment, the best is to modify the HikaShop core to add your code.
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.