You would need Virtualmin Pro in order to create a reseller account. Resellers are only available in Pro, it’s not a feature available in the GPL version.
Is it possible to add VPS servers through Virtualmin + webmin
No, Virtualmin is more for managing a single shared server.
However, you may want to take a look at Cloudmin. Cloudmin can create VPS servers.
The new Cloudmin GPL can manage KVM instances for free, if you’d like to try that out: