custom: how do I augment an option?

[Sam Steingold <sds@gnu.org>]

If I want to set a custom variable, I can do
--8<---------------cut here---------------start------------->8---
(custom-set-variables
 '(foo 42))
--8<---------------cut here---------------end--------------->8---
in my .emacs.
However, if I want to modify the custom variable, I have to resort to
something like
--8<---------------cut here---------------start------------->8---
(add-hook 'message-load-hook
  (lambda ()
    (add-to-list 'message-syntax-checks
                 '(long-lines . disabled))))
--8<---------------cut here---------------end--------------->8---
because
--8<---------------cut here---------------start------------->8---
(custom-set-variables
 '(message-syntax-checks (adjoin '(long-lines . disabled)
                                 message-syntax-checks
                                 :key 'equal)))
--8<---------------cut here---------------end--------------->8---
will evaluate `message-syntax-checks' too early:
--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (void-variable message-syntax-checks)
  (adjoin (quote (long-lines . disabled)) message-syntax-checks :key (quote equal))
  eval((adjoin (quote (long-lines . disabled)) message-syntax-checks :key (quote equal)))
  custom-initialize-reset(message-syntax-checks (if message-insert-canlock (quote ((sender . disabled))) nil))
  custom-declare-variable(message-syntax-checks...
  byte-code...
  require(message)
  byte-code...
  gnus-msg-mail(nil nil nil nil nil nil nil nil)
  compose-mail(nil nil nil nil)
  call-interactively(compose-mail nil nil)
--8<---------------cut here---------------end--------------->8---
(it also uses a CL function which emacs purists frown upon).

So, how do I use the custom facility to add something to a custom variable?
Thanks!

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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