Merge Individual Buses to Main Bus instead of Nesting

My requirement is to create a bus structure such that it passes through every subsystem in a flow without creating nested bus structure (i.e adding new bus structure to existing bus structure). Instead all the buses shall come on main parent level instead of being nested under each other.

The new signals (combined as a new bus) coming from new subsystems should be appended to existing main bus structure to avoid too much nesting.

How can i achieve that?