at this moment you can make that pool via truenas ui as a one drive "stripe" pool
however this causes hexos to freak out at the time and i wouldn't recommend it.
i too am waiting for hexos to not freak out when adding one drive pools since i would like a single ssd to be my app drive
The team is aware we want single drive pools and it will be coming in a future update.
At this time there is no eta