Hi, Chinna.
I've got your problem.
I've thought you didn't see this property so you couldn't change it to the desired value. But your problem is a limitation by design of CP: "This composite provider can be added..." is available for CP containing UNION parts only. You could hover the mouse over the checkbox, and it will give you this info (a tooltip for checkbox).
So you had to adjust your design and nest CompositeProviders to join unions: the topmost CP must be a join in this way and it will not be stackable.
The value of mentioned field in table RSOHCPR is the value of this property only: it doesn't enable this setting, but just contains its value. The row of this table describes the properties of the selected CompositeProvider.
If you want to use more complex modelling scenario, you can create calculation view in HANA and use joins, unions and aggregations in nearly any combination. Then you can consume this model via CompositeProvider.
Hope I was helpful.
Regards,
Andrey.