How can we install sshpass to help automate SSH related tasks?
TrueNAS is an appliance. You are not expected to be able to install random things on the appliance platform. The appliance should be considered to be a single package that comes as distributed, take it or leave it.
If you try to force changes to the platform, you can probably succeed. It's an open platform, after all. However, changes may not survive updates, or may create problems, crash your system, kill puppies, and/or cause the end of all life in the universe.
If you wish to disregard this, you install things by writing files to the places that they're supposed to go, and then testing, and then making sure that it hasn't broken any obvious functionality.