I am trying to model a polymer system using coarse grained soft potential. The potential is dependent on number density of particles at a point. However for nonbonded interaction calculation, the particles are not point particles but are regularized using weighted-density approximation around the point. That is to say particles are like soft balls with radially decreasing density as we go towards it surface (density 0 at surface). As I understand, I need to add new type of potential to LAMMPS. I am wondering if LAMMPS has a built-in function to calculate weighted density that I can use in the code.

I am trying to model a polymer system using coarse grained soft potential.
The potential is dependent on number density of particles at a point.
However for nonbonded interaction calculation, the particles are not point
particles but are regularized using weighted-density approximation around
the point. That is to say particles are like soft balls with radially
decreasing density as we go towards it surface (density 0 at surface). As I
understand, I need to add new type of potential to LAMMPS. I am wondering if
LAMMPS has a built-in function to calculate weighted density that I can use
in the code.

no, but what you describe sounds somewhat similar to the embedded atom
model (EAM).
for those calculations, you have two steps: 1) you compute the
(electron) density contributions at the point of the atom and with
this you compute 2) an additional term to the (static) pairwise
interaction. so you may want to look at the ::compute() method, as it
shows you how to set this up properly in parallel.