There is a report called' lateral stability of steel beam' or similar names.
It gives example for this kind of condition, it does not suggest to hange the timber joists to the top flange, you should use timber block fixed on the bottom flange and sit the timber joist onto the block.
Further...
I am calculating the dynamic response of a steel frame structure with composite beams. When I use section designer, I input the concrete flange in. The width of the flange was calculated by hand. I compared the result of the steel frame with these beams and the result from beams without concrete...