Incorporating new things into completions

[Psionic K]

I want to augment `switch-to-buffer' candidates to contain a list of
exotic fruits.  Presumably the downstream handler for buffers has no
idea what to do with fruits, so I need to provide some alternative
handler.

Has this infrastructure been built into Emacs or is it exclusive to
completion packages at this time?   If Emacs provides facilities for
this kind of thing, what vocabulary should I become familiar with?