Hello,
I'm sorry ; Since HikaShop or HikaSubscription requires a logged user to authorize a download, the Joomla update system won't be able to perform a download.
Thus, it would require a custom plugin to override the download system, read a token from the URL and then authorize the download.
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.