The llapi_layout_comp_add(layout, new_component) function should inherit unset fields from "layout" if they are unset:
adds the layout component comp into the layout.
The layout will be turned into composite if it was plain before adding. All layout attributes of comp will be inherited from the layout, unless otherwise specified for this component by llapi_layout_*_set(3) functions.
In my testing, the "--pool" option is not inherited for the second and later components.