I see one way of doing this, Jesper.
I requires you to have the layer twice in the Layer Control and split the Label Expression into two. But it will also have to have the label shown on two lines - I hope that is ok?
For the first layer, set the label font to black and the expression to: Outer_dia+" "+Dia_unit+" "+Material
For the second layer, set the label font to read and the expression to: Chr$(10) + "(" + Label + ")"
Let me know if this works for you