Don’t do it Like 3 posts in the last week or two with people unable to make pools on USB docks, long time known & unsupported.
What motherboard are you using? Any chance you have any pcie slots that can use a real HBA? If not, I guess the least improper way would be move all data to single disk, create raidz2 pool, copy data to raidz2 pool, destroy single disk, expand raidz2 pool. 98% of those steps can be done using the GUI at the very least…
Like 3 posts in the last week or two with people unable to make pools on USB docks, long time known & unsupported.