I don't know how you define a "rectangular" hole in Easy-PC, but it needs to be milled by the PCB Fab anyway, and defining a "throughplated" milling contour as a separate gerber layer is most likely the best way. Alternatively, you can inform the Fab, that a particular unique drill diameter is marking a slot cutout. In any case, you should ask them in advance about their requirements for throughplated slots. The option is most likely not provided for pool fabrication of PCBs.