Constraint relative arrival time for a group of signals
One way is to define a FROM TO constraint, e.g. in UCF syntax:  

TIMEPSEC "TS_PAD2FF" = FROM PADS("my_ports") TO FFS("my_ffs") 4 ns;


However, this does not define the maximum allowed difference of the path delays, but an absolute path delay for all paths from a group of pads to a group of FFs. So a possible fixed timing delay which all paths have in common is not considered in this constraint. It's as close as one can get...
If it was possible to define a maximum skew for a general group of paths that would be another option, but all tools I have come across only allow skew definition with respect to a clock signal.