I think you just need to make the DataType field of the element that is another bus equal to something like 'Bus: elementEp_struct_Ex000_struct'. For example, a simpler version:
bus1 = Simulink.Bus;
el11 = Simulink.BusElement;
el11.Name = 'myel11';
el12 = Simulink.BusElement;
el12.Name = 'myel12';
bus1.Elements = [el11 el12];
bus2 = Simulink.Bus;
el21 = Simulink.BusElement;
el21.Name = 'myel21';
el21.DataType = 'Bus: bus1';
el22 = Simulink.BusElement;
el22.Name = 'myel22';
bus2.Elements = [el21 el22];
The resultant structure should be:
struct bus1 {
double myel11;
double myel12;
};
struct bus2 {
bus1 myel21;
double myel22;
};