Well:
FreeNAS' kernel may or may not be compatible with it's FreeBSD alternative. So you may have to get the source code for the hardware and compile it in with FreeNAS. This means you must:
1. Create a build environment to compile FreeNAS yourself, with the appropriate driver added.
2. Have access to the source code.
For many people, both of those are virtually impossible to accomplish.
I take the stance that if the drivers aren't part of FreeNAS already, you probably shouldn't be trying to use them.