It can be done from the reflector side by melting a passage for bulb like wpoll said. Or like on the found picture grinding or remelting with for example soldering iron the melted points on the sides of the base connector piece.

Or like many do just replace the whole thing with LED alternative for whiter colour.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey