Add strings to zsh tab completion for all commands and arguments

I want to add the keywords foo and bar to be added to the auto completion results for any command or argument.

eg:

cd <TAB> should give foo and bar in the suggestions in addition to the existing completions

blah <TAB> should give foo and bar in the suggestions in addition to the existing completions.

The zsh completion articles i have been reading mention methods to add completions to specific commands, but none mention how to add a set of words to global completions ( or if there is a global completion list which always exist )