Adding a computer to the local machine's list of PowerShell TrustedHosts

Retrieve the current list:

$curValue = (get-item wsman:\localhost\Client\TrustedHosts).value

Set the new list, appending the new host onto the old list:

set-item wsman:\localhost\Client\TrustedHosts -value "$curValue, Server01.Domain01.Fabrikam.com"