A lookup table where one value is unknown

I have a field in SQL table where the column will contain a pre-determined text string  and I want a scribe lookup table where it will take that text string and lookup a corresponding value. Normally that is fine, I can create lookup tables. However, on some instances the text string passed through will not be recognised and in those circumstance I don't need it to use the lookup table at all, I can just use that text string. How would I go about this​? Essentially, if the value is in the lookup table then use the corresponding value else use the text string itself.

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