As of Simulink 8.1 (R2013a), it is possible to create a Simulink bus in which one of its fields is an array of buses. However, it is not available in Simulink R2015a to initialize a Simulink bus that contains a field which is an array of buses. As a workaround, you can create the buses separately, concatenate them using the Vector Concatenate block, and then include the array of buses in larger bus.
I have attached a zipped folder 'example.zip' for this workaround. The model 'sub_bus_example.slx' uses the Simulink.Bus objects that are defined in the 'busInfo.mat' file, and the 'makeStruct.m' script shows how to make these Simulink.Bus objects. Note that you can nest this structure further.