I think I'm one step further. After the read-data action I've added a PHP-action, as suggested by Max.
I can use this statement to convert the data to a proper array-format:
explode("|*|", $this->get('gebruiker.cb_lidmaatschap'))
I've tested this code in the PHP-action:
$var = explode("|*|", $this->get('gebruiker.cb_lidmaatschap))
foreach ($var as $value) {
echo $value;
}
Then all values are displayed.
If I know how to get this result into a multi-checkbox formfield, I've got what I want.