You'll probably need to change the device rules for the jail. I don't remember which file that is off the top of my head, but it will mean making the root filesystem on FreeNAS read-write so you can edit it. Probably in /conf/base/etc somewhere.
The next problem is whether the fusefs-ext4fuse package requires any special kernel modules or access, which could be a show stopper for this plan.
Don't try and install that package outside the jail or you're asking for problems. How much data are you talking about transferring? You might get things done a bit faster sharing it from another system and copying it with rsync over the network.