ExternalCall to PS - Best practice for setting variables

I want to dynamically build a list in UI++. My thought was that I would call a powershell script that would set an environment variable to comma separated list of choices, then I would call "ChoiceList" with "OptionList" set to the environment variable, but its not working.

Right now I am testing outside of a TS so I am just using "regular" environment variables.  If I change a variable with an externalcall does UI++ pick it up or does UI++ only scan the values at startup?

Example:

%Sources% = "" when UI++ is called

Part way through, if a checkbox was selected, then I will perform an externalcall

<Action Type="ExternalCall" Title="External Command" Condition='"%Restore%" = "True"'>powershell.exe -ExecutionPolicy Bypass -File GetSources.ps1 -NoFile</Action>

Now the environment variable "Sources" to a comma separated list  "SerialNum1,SerialNum2,SerialNum3"

Then I hit these directives:

<Action Type="Input" Name="RestoreSource" Title="Restore Profile" Condition='"%Restore%" = "True" AND LEN("%Sources%") > 0'>
   <ChoiceInput Variable="RestoreSource" Sort="True" Question="Select A Profile To Restore">
      <ChoiceList OptionList="%Sources%" AlternateValueList="%Sources%" />
   </ChoiceInput>
</Action>

Is there a best practice for setting the environment in PS so that UI++ sees it?  Even if I get %Source% to populate, would this even work?