[TYPO3] Extend Flexform

[Juan Madurga]

Hi,
Im extending an extension which has flexforms. I want to add some more 
parameters in the flexforms but i dont know exactly.
I have modified this files:
          flexform_ds_pi1.xml
          locallang_db.php
          locallang_db.xml

I saw in a tutorial that i have to modify the ext_tables.php file.
In that file i have:
t3lib_div::loadTCA("the_extension_i_extend");
and then:
t3lib_extMgm::addTCAcolumns(..)
t3lib_extMgm::addToAllTCAtypes
to add new TCA.
I have tried to put:
t3lib_extMgm::addPiFlexFormValue($_EXTKEY.'_pi1', 
'FILE:EXT:new_exten_extension_path/flexform_ds_pi1.xml');
But it doesnt work.
Also so i read in the tutorial:
$TCA['tt_content']['types']['list']['subtypes_addlist'][$_EXTKEY.'_pi1']='pi_flexform';
With no result.

What i have to do to add or modify the flex form of an extension i want to 
extend?

Thak you very much.

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