how to extend `C-x 8` keymap?

[Tom Roche]

Apologies if this is a FAQ, but I didn't see it in the GNU Emacs FAQ or while 
googling (and I have an `info` problem, about which separately):

How can I (presumably in init.el) extend my `C-x 8` keymap? E.g., I want

          to
sequence  produce  as does (currently)
--------  -------  ------------------------------------
C-x 8 d   δ        C-x 8 RET GREEK SMALL LETTER DELTA
C-x 8 D   Δ        C-x 8 RET GREEK CAPITAL LETTER DELTA

Currently those sequences are undefined.

TIA, Tom Roche <Tom_Roche@pobox.com>