If you are asking about how to have diskless hosts to join with your existing (one?) PVE deployment into a cluster, then you would need to basically manually make Debian install booting over PXE (and writing its content back over e.g. NFS on shutdown) with sufficient amount of RAM to be able to run your other guests. But you would run the main issue that should you want to reboot your whole cluster, everything would be dependent on your guests providing PXE boot (and e.g. NFS share) and thus that one "main" host.
If you just want those "other computers" to be diskless stations, then depending on the OS of your choice it would be doable in analogous fashion without the caveat mentioned above. You need to look for solutions for your particular OS you want to have running there.
If you wanted some sort of hybrid where a VM would be using CPU+RAM of a diskless external machine and also show as a VM in PVE of another host, I am not aware of any hocus pocus like that in PVE, but once you have e.g. Wake-on-LAN possible on those stations and PXE boot, you can remote manage them in many other ways.
