Right, I don't think you can "import" a powershell script into a ruby script (at all, in any situation), but you can call the script directly from the ruby script with something like:
$result = `powershell.exe ../../testfile.ps1`
Having the file in your environment is mostly useful for being able to load it through Puppetserver and being able to reference it by `module/name`, but when PE goes to run the task it will upload it to the remote machine to a temporary directory to execute it. By specifying the file in your task metadata, PE will *also* upload the required file alongside the task file so it's in a predictable location and can be used on the remote system.