in 0.9 or the current nightly builds you can add addons from anywhere, just add the relative path to the addon from the project to addons.make like:
ofxGui
../addons/ofxLocalAddon
that will add ofxGui from the addons folder in the OF install and ofxLocalAddon from a folder one level above the project.
another way is to add in config.make
PROJECT_EXTERNAL_SOURCE_PATHS = ../../../someCodeFolder
which will add recursively all the code in that folder but it's sometimes problematic and doesn't parse includes, libraries... as well as including an addon