This is one I haven't heard of. I know you can use busses inside the block, but I thought that the block itself had to have I/O pins in order to connect to the external circuits. If you need to use a bus to interconnect the different blocks you can use one outside the bliocks and run all the hierarchy block I/O pins to the bus.
I am using Multisim Power Pro 2001 so it may be different in a newer version. I may be misunderstaning your problem and if I am I apologize in advace.