How to route a node from one block to another block in Altera Quartus II
Make sure any nodes you'll want to access from another block are
connected to input or output pins in the BDF
You didn't say what version of Quartus you're using, but somewhere under the file menu, there will be a dialog along the lines of "Create block symbol file". This will run a compile, and generate a .bsf file of your current BDF.
You can now add that BSF as a block in another BDF.


If this question doesn't get closed, I might come back and expand this answer to include some graphics.