Block Design file issue

Usually, if you already have a bdf/bsf file, it would be good to connect them in top level bdf file. You could create block symbol for all other design files (written in any HDL) using "Quartus II > FILE > Create/_Update > Create Symbol files for current file" and then connect them in top level bdf file as is recommended by hapyang.

Alternatively, if you want to go HDL way, you can generate VHDL file for the bdf/bsf file using "Quartus II > FILE > Create/_Update > Create HDL Design file for current file" and then use the newly generated HDL file along with your code.

Although both methods can be used, I feel that 1st method should be more effective.