Hey y'all. Doing this natively as just UNRraid is possible since Slack can cluster. You'd have to dig hard into the console and install things like slurm to manage the workload. You're not going to get the nodes to present in the UNRaid GUI properly though.
However, if you want it to be much easier on yourself, set up a ProxMox cluster and make a UnRaid VM on it. You can assign UNRaid all the CPU cores and memory you want, and even pass through storage for UNRaid to manage.
I love UNRaid. Keep thinking differently y'all.