How to set environment variables for system extension

Hi Experts,

I knew there is LSEnvironment for defining environment variables to be set before launching App.
e.g.
<key>LSEnvironment</key>
<dict>
<key>PATH</key>
<string>/Users/flori/.rvm/gems/ruby-1.9.3-p362/bin:/Users/flori/.rvm/gems/ruby-1.9.3-p362@global/bin:/Users/flori/.rvm/rubies/ruby-1.9.3-p326/bin:/Users/flori/.rvm/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:</string>
</dict>

How about system extension? How can I set some environment variables before running? Thanks a lot.