I think you will require spawning particules in the middle of the domain and that will require some internal macros which are not supported. So is not something I can discuss or evaluate on a public forum. Stick first to what is built-in.
Another idea is to write a file injection every time step and update an Injection based on File with that.
Or you inject dummy particles as place holder (so very small particles, almost massless) and these particls will gain main based on your criteria in cells.
You are navigating away from the AIS Discovery experience