At least FreeNas 9.10 does, via iohyve/bhyve [0]. I installed a Ubuntu Server image using it the other day and it worked flawlessly.

[0] Docs are a little spartan, but google searches got me up and running a virtualized linux image in less than 15 minutes. https://doc.freenas.org/9.10/jails.html#using-iohyve