Are you asking if you can combine two different kinds of projects, a CCS project and external makefile project? That is not possible, but if you already have an external makefile you are using for build, you can create a Makefile project in CCS that uses that makefile instead of the one that CCS auto-generates.
If on the other hand, you are looking to simply import the code, you can do that with a CCS project and let CCS auto-generate the makefile to use for the project build.
http://processors.wiki.ti.com/index.php/Projects_and_Build_Handbook_for_CCS#Types_of_Projects