New function t3lib_extMgm::addFieldsToAllPalettesOfField() allows to add fields to palettes of an existing field. It can be called like:
/**
* Adds new fields to all palettes of an existing field.
* If the field does not have a palette yet, it's created automatically and
* gets called "generatedFor-$field".
*
* @param string $table: Name of the table
* @param string $field: Name of the field that has the palette to be extended
* @param string $addFields: List of fields to be added to the palette
* @param string $insertionPosition: Insert fields before (default) or after one
* of this fields (commalist with "before:" or "after:" commands).
* Example: "before:keywords,--palette--;;4,after:description".
* Palettes must be passed like in the example no matter how the
* palette definition looks like in TCA.
*/
t3lib_extMgm::addNewFieldsToAllPalettesOfField($table, $field, $addFields, $insertionPosition);
More info:
RFC: Feature Request #9625: Integrate possibility to extend TCEforms palettes easily
http://bugs.typo3.org/view.php?id=9625