Thank you for the suggestion. I modified the "overrides" element of creality_ender5plus.def.json adding :
"prime_blob_enable": { "enabled": true, "default_value": true },
"extruder_prime_pos_x": { "enabled": true, "default_value": 30 },
"extruder_prime_pos_y": { "enabled": true, "default_value": 30 }
but with only a partial success: now "prime_blob_enable" is visible in settings and can be enabled or disabled, but both the position elements X and Y have no effect: they can be checked in the Visibility setting panel, but are never shown in the actual setting. Moreover, the default values are ignored.