The 9720 has a Thunderbolt port, so will lose out on the speed and features (such as charging the system) of that if you use an old docking station that's USB only, and be limited to video output and resolution by the docking station.  As long as you don't need more features than you get from the old docking station, it should work fine.  
Hi; I just purchased a XPS 17 (9720) laptop. I have a docking station for my old Inspiron. It connects via the old/original USB connector while the XPS has only the new USB-C connectors. Can I use this docking station with my new XPS? And is there any downside to using the old docking station? The old station is a D3100. thanks - dave
Can my new XPS laptop use my old docking station?