In the meantime I got some progress.
In a former project I needed to add some extra fields from custom post types to the admin mail. So I added in lib-aux.php ahter the “Excerpt”_replacement:
$m = str_replace( '{Excerpt}', $post_excerpt, $m );
// DC: Hook zum Einfuegen eigener Variablen
$i = $page;
$m = apply_filters( 'cforms_apinst_email_filter', $m, $pid, $i );
and hooked the filter in my functions.php
add_filter('cforms_apinst_email_filter','apinst_email_filter',10,3);
to do somthing like
function apinst_email_filter( $mailtext, $pid, $i ) {
// get custom fields
$custom = get_post_custom($pid);
$myvalue=$custom["_myfield"];
$mailtext = str_replace( '{myplaceholder}',$myvalue, $mailtext );
return $mailtext;
}
In the actual project I have to do something similar when the form is rendered. I need to read the available values for a checkbox from a list of custom post types. After some research my approach is to put the hook in cform.php after line 355, so i can read and modify each field value.