zpool import should show the pools that the system can see that haven't been imported into the system. So, if all is well, then your raid2 pool should be listed when you run
zpool import. If it is, then you can run
zpool import raid2 to import it. Odds are it'll complain that another system is using it, since I'm guessing that you never exported it. If so, then just use the
-f to force it.
This won't be required when upgrading TrueOS, but going from PC-BSD to TrueOS is more like a fresh install than an upgrade except that it puts the new OS in a fresh boot environment instead of blowing everything away, so your root pool and your home directory are fine. But you still lose most of your settings in the process, and the list of zpools appears to be on that list.