I don't think that this will be possible - you will need to go the extra step and create (and probably delete) these variables within the script - for this could be include-variables quite helpful: The $(Include) which you $(Must_Include) into your toolkit.
michielvandegoor, sub parameters are only available within the sub you're using them.
Is this something you can use :
SUB ListFiles(pLocation, pReturnValue)
// Do something
LET $(pReturnValue) = '$(pLocation)';
CALL ListFiles('C:\downloads', 'vLocation')
After using this call statement there is a variable with the name 'vLocation'.