This is best done at the data layer, so within you MySQL DB. However, you can connect to the MySQL DB and connect to your spreadsheet, then blend the two on the Symbolic Name (KC_5_1). You would then use the content name field from the spreadsheet (FAQ) and bring in data from the MySQL connection via a blend. When blending, you will not be able to dis-aggregate below the level of the blend.
This article provides a good example on how to set up a blend: Data Blending with Summarized Data | Tableau Software
Hope this helps!