You can replace As with P and do the topology generation using  any tools/server based on your ff,then replace the P with As and change the parameters that you have to get from QM calculation such as bond angles,charge etc .

i think you have to make appropriate change in the respective ff parameter files.!

You might able to get some suggestion from Gromacs mailing list too..

topology files can be found online and downloaded. you can use according to the molecules of your interest. I downloaded the attached file from internet to use for simulation via NAMD. but i am not sure if they will work work with your compound but you can try!

thank you manisha and nikhil