Proxmox - NixOS

Hi,
yes it should be possible, but you won't get the functionality of that distribution outside of that VM.
From the description on the website it sounds like it won't be easy to export a VM generated by that OS to the outside (PVE):
This builds and starts a virtual machine that contains the new system configuration (i.e. a clone of the configuration of the host machine, with any changes that you made to configuration.nix). The VM doesn’t share any data with the host, so you can safely experiment inside the VM. The build-vm command is very efficient (it doesn’t require a disk image for the VM to be created), so it’s a very effective way to test changes.
but I might be wrong about this.

Best way to find out is to experiment ;)