Negative set-up time just means that it is permissible for the signal to arrive after the clock edge and there is usually a corresponding increase in hold time.
It can sometimes be seen in situations where there the clock path has a much longer delay than the signal path (between the point where the setup hold time is measured and the point where the signal is actually sampled).
just imagine a flop with some buffers at the clock input that delay the clock signal. now wrap all in a nice wrapper and call it a flop. as mentioned above this "new" flop can have negative setup but its hold will increase.
How can u get some buffers in clock path if you design your flip flop without those buffers and send them for fabrication. Again after fabrication these flops are charecterized with their set-up, hold etc. and put it into the .lib file.
you asked for A way - and I tried to give an example of A possible way this can be done.
it is just illustrative example, I don't say you should go and design like this now
When the clock signal transation time is too slow and the data signal transation time is too quick, the setup time will be negative. And the setup time + hold time >= 0. The total of setup time and hold can be view as the data must be stable period. If the setup time decrease, the hold time will increase. You can image that the timing window is just shifted.