ZFS Raid extension

So I am currently building my Ugreen nasync dxp4800(4 possible bays) and while I got my ssds ready, I still need some HDDs. However my setup was already quite expensive and I want to start with 2 or 3 HDDs and add the rest later. How easy is it to add a drive later and add it to an existing pool and is this already possible in proxmox? Which raid level would you recommend?