I am just a beginner in vhdl language. I am using quartus 2 (version 7.2). I designed a vhdl code and block diagram with separate projects for one design. But I can't combine codes and blocks. Is it possible to combine them into one design?
Yes. You can have a block diagram as design top using subblocks written in VHDL and also use schematic entry for components in instantiated in VHDL.
The former method (schematic top design) is probably used more often. Look at the menu functions under File\Create/Update for creating a symbol file for a VHDL design that can be imported in the schematic entry.