I patched the file "manage_proj_ver_add.php" in order to create unreleased projectversions by default.

old:
version_add( $f_project_id, $t_version );

new:
$t_default_version_release_status = config_get( 'default_version_release_status' );
if (isset ($t_default_version_release_status)){
version_add( $f_project_id, $t_version, $t_default_version_release_status );
}
else {
version_add( $f_project_id, $t_version );
}

Now it is possible to set the behaviour via config_inc.php:

// Possible values see constant_inc_php (Versions): VERSION_FUTURE, VERSION_RELEASED 
$g_default_version_release_status = VERSION_FUTURE;