First you'll need to copy the driver file across.
Easiest way for me was to ssh into the box, then transfer the file via sftp.
Just remember to first run
[PRE]mount -rw /[/PRE]
Otherwise it won't work because the filesystem is be default mounted as read-only
Once you've got the file across, run:
[PRE]pkg_add aac9x-amd64.tgz[/PRE]
then reboot and enjoy.
A word of caution:
https://bugs.freenas.org/issues/2502
I don't know what the issues are with it, but I'm effectively using it in HBA mode, so fingers crossed it won't be a problem.