I have never flown metric altitudes, or the A380, but surely like other Airbusses, the A380 FCU ALT selector has a metres button, so no conversion from feet to metres is required - just press the button to change the ALT and PFD readouts to metres and then select the metric altitudes that ATC or the approach plate says?
Secondly, the elevation of the airfield is taken into account in the box on the approach plate of altitude versus DME, so you just read off the numbers to cross check - especially important when doing a non-standard approach, such as intercepting the GS from above.