Unfortunately, what you are trying to do is not possible in ARCHICAD at the moment, because Building Material Properties cannot be Expression-defined. Probably this is the reason why the Dialog says "Not applicable". This is a current limitation and I don't know if there are plans to develop it or not.
You can make a wish for this feature, we can vote on it and if it gets enough support, we can submit it to GRAPHISOFT for consideration.
If you do make a wish, please post its link in this thread.
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