It's a miracle! I googled a typo3 problem and actually found an answer that worked! I needed an additional field in tt_content, so first of all I added a new field called "tx_kurs_id" to the tt_content table in the database. After that I added the following to the ext_tables.php in my extension:
$tempColumns = Array ( "tx_kurs_id" => Array ( "exclude" => 1, "label" => 'LLL:EXT:pgk_kurs/Resources/Private/Language/locallang_db.xml:tx_kurs_id.kursid', "config" => Array ( 'type' => 'input', 'size' => 30, 'eval' => 'trim' ) ), ); t3lib_div::loadTCA("tt_content"); t3lib_extMgm::addTCAcolumns("tt_content",$tempColumns,1); t3lib_extMgm::addToAllTCAtypes('tt_content','tx_kurs_id','','after:section_frame');
The last line is the interesting one.
param1: the typo3 table that the field should be added to
param2: the name of the field (as defined in $tempColumns)
param3: which content elements to add it to (leave empty for all elements)
param4: where to place it