I afraid it's not an option for 5600.  Early on thunderbolt add in card was made available with these systems 5810/7810/7910/1700MT/1700SFF/3620MT and 7910 rack. 
DELL PRECISION T5600 WORKSTATIONCan I install a Thunderbolt card? I want to connect a Mac monitor to my DELL PRECISION T5600 WORKSTATION.I think you need to install a Thunderbolt card to use it.Is there any way? 7HMP DELL Thunderbolt-2 PCIE Can I buy this model?
DELL PRECISION T5600 WORKSTATION Can I install a Thunderbolt card?