Reading the wiki would help to understand C::B's own build system, it's not makefile based.
http://wiki.codeblocks.org/index.php?title=The_build_process_of_Code::BlocksYou can use a makefile, the main makefile should be placed in the projects root directory.
To use a makefile-based project with C::B, just create an empty project with the wizard, place your source-tree with the main-makefile inside the projects root-dir and check "This is a custom Makefile" in the projects properties.
Make sure the make commands used by C::B are set correctly for your project (see projects build options tab "Make commands").
And of course you can search the forum, there are some threads about this, especially in the last time, because of changes to the makefile-based build-system.
C::B does not ship with a makefile generator. But you can search the forum for cbMakeGen (or cbMekfileGen I don't remember exactly).