It is not recommended to place any conductive material as overlay on top of the sensors for the following reasons:
1. If the metal object is floating and is not grounded, then accidental grounding of the metal would lead to raw count saturation and is a potential false trigger.
2. If the metal is ensured to be grounded at all times, then the sensitivity of the button would drastically decrease owing to increased Cp.
If you are looking for metal over touch applications (metal overlay), we recommend you to look for Inductive sensing based touch detection that we offer. For more information see Inductive sensing app note.