Thank you for your reply. Anyway, that's not what I asked. I already created an environment.plist, first manually, then with the DevTools property list editor.
What I want to know is:
How is the syntax if I want to append something to an existing environment variable?
The bash shell syntax would be:
PATH=$PATH:/usr/local/bin
However, if I put a value like '$PATH:/usr/local/bin' into environment.plist the variable is literally set to this value, i.e. $PATH is not expanded to the contents of the existing value of the PATH variable.