[Typo3-dev] How to make use of extended TCA?

[Simon Liebold]

Hello, 
what is the right way for writing into DB-fields that weren't configured
in the TCA? This is one of my first looks under the hood of Typo3 -
please excuse my lack of experience.

I used this example 
http://typo3.org/documentation/document-library/doc_core_api/columns_fieldnam-8
to extend the TC-array. Does this have do be in a extension's tca.php or
ext_tables.php? I tried both. :-|
My changes show up in the tools-->configuration-->TCA tree but still the
field remains emty after trying to fill it. However, it works if i
change t3lib/stddb/tbl_be.php itself. I know, this is not the proper way
to do it ;-)

That is, what i have in the extension's tca.php (i am trying to mess
around with sys_action):

$tempColumns = Array (
	'usergroup_cached_list' => Array (        
		'exclude' => 1,        
		'config' => Array (
			'type'=>'passthrough'
		)
	)
);
t3lib_div::loadTCA('be_users');
t3lib_extMgm::addTCAcolumns('be_users',$tempColumns,1);

Background and why i want to write to be_users.usergroup_cached_list:
http://typo3.org/documentation/mailing-lists/english-main-list-archive/thread/110112797/?tx_maillisttofaq_pi1%5Bpointer%5D=1&tx_maillisttofaq_pi1%5Bmode%5D=1

What should i do?
-simon