WD19S supports mac address pass through May be this may help [URL]  
Hi,  I have about 50 docking station, Dell Dock – WD19S,  when I connect the docking station to the laptop the mac address is the same that the on the bottom of the equipment, however after I install the docking station driver, the mac address change completely, I just saw this problem this week, and most of the laptop I use is Latitude 5420. Please don't tell me the MAC is flashed onto the network adapter and can't be changed, because I repeated the test 5 times to make sure that I wasn't crazy.    
Docking station MAC address has changed