Adding options to application, upon startup?
You can change the option temporarily by quitting Chrome if it is already open and running either of these commands:

open -a Google\ Chrome --args --enable-easy-off-store-extension-install

/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --enable-easy-off-store-extension-install

You can change the options permanently by replacing Chrome's executable with a shell script or by using another application or script for opening Chrome. See this question.