You'll want to use Data Blending. With cube data sources, the cube will always have to be the primary data source, and you can't blend 2 cubes. Seems like you have a cube and a flat table, so this last point should be fine.
To learn more about data blending you can check:
- Video: Data Blending
- Article: Blend Your Data - Tableau
The cube must have an underlying data source which is a relational database. If you can connect directly to it, things are going to be much simpler.
Hope this helps!