Why do you want to import VMs from a different hypervisor? Why not build an 'golden image' on your Proxmox machine, and create a template out of it.
This is basicly how I create my new VMs. I keep a relatively updated 'golden image' in my library and clone that to a VM as many times as I need. To be fair, I'm running ESXi. I have no idea if Proxmox even has this as a feature.