Assuming the existing code has a build system like Make or CMake, etc., you probably want to use the “External Build System” template in the “Other” section of the project panel. If you want to include existing code in a new application / framework / etc., create a template for the type of product that you want to eventually build, and then add the “Run Script” build phase to invoke the external build system.
MacResearch.org has a tutorial on using Xcode Organizer to create external goals that may also be useful.