Howdy Tony,
Currently as of TestStand 3.5, you can't enter an expression in for the Expected String Value option. This has been sent in as a product suggestion to our R&D team.
However, one workaround is to use a Pre-Expression to set that string value property. So, in your Pre-Expression for that string test step, you could say Step.Limits.String = ThisContext.Locals.CompHex.
Hope this helps!
Best Regards,
Jonathan N.
National Instruments