You need to change the excecution policy first for Powershell. You can do locally on pc for example. set-executionpolicy -ExecutionPolicy remotesigned. Then you just create a PS task in Altiris. It will execute it according to the policy.I've done globally via GPO.
If you run the script as parameter from the command line, you can specify the execution policy at runtime.
For example:powershell -ExecutionPolicy ByPass -File C:\myscript.ps1. It will run regardless of the pc's execution policy.
At https://powershell.org/ you can ask any powershell related questions.