How to convert asynchronous set FlipFlop Hard macro into asynchronous reset FlipFlop?
If you have an asynchronous set flip flop (with inputs D,S, clock C, and output Q), you can make a an asynchronous reset flip flop simply by inverting D and Q.


async_set_FF_S := async_clear_FF_R;
async_set_FF_D := not async_clear_FF_D;
asunc_set_FF_Clk := async_clear_FF_clk;
async_clear_FF_Q := not async_set_FF_Q;


you may need to take care with global resets (which are now global sets).
I'm surprised your synthesis tools don't have an asynchronous clear flipflop, what is it?