Hi,
HikaShop have an integration with the component "Akeeba Subscription", it allows HikaShop to have some renewal payments. We also have an integration with "paypal recurring".
About HikaSerial, it will have no problem to generate your vouchers. But if you use akeeba subscription, it will generate one order per payment and HikaSerial will generate one serial per order. So in that case, you should just set the validation for one month on the voucher.
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.