Static and Dynamic value for a drop down simultaneously

Dear Experts,

I would like to explain my requirement with an example as below:

When the application is loaded, DROPDOWN_1 should display a value from DS_1 which is dynamic. Based on some calculations, default value is selected as below:

Now when we drill down the drodown, it should show the values from a static list like below:

This is confirmed that DS_1 will populate a value which will definitely be present in static list.

Now I am able to populate default value from DS_1; but in doing so, I am getting all the values in DROPDOWN_1 from DS_1 only. I am not able to get the further drilldown values from static list.

I could not find any help regarding this on SCN. Kindly help.

Thanks and Regards,

Nitesh Kumar