Hi,
There is no built-in import system in HikaShop for jStore / jMarket.
But depending what exactly you want to import ; I think that some parts could be done using some SQL queries.
It would require an analyze of Joobi components but it will highly depend on your needs because more you might want to import (different element) more complexity it will be.
Also are the registered members considered as vendors? or is there a accreditation process?
Both modes are handle by HikaMarket.
You have the "automatic vendor creation" mode in the configuration and you can use Joomla user groups for the "accreditation process" feature.
Regards,