It is correct that Incus OS doesn’t support unencrypted pools. This has been asked before and a possible procedure was discussed in Installing IncusOS and retain / reuse existing pools? It unfortunately requires to have enough space on a second drive to move your data around.
Alternative you can of course reimport the pool into Incus but in this case it stays unencrypted.
ZFS doesn’t have a feature to convert and encrypt an existing pool. Would be a cool feature…