Is it possible to code custom particle force?

Questions & Answers
1

Example: wind, drag field, goal ...

2

yes it is possible!

You could create a script and attach it to your particle system gameobject and use the particleEmitter object for modifying your particle system.

Here is a small example for setting the x value of the localVelocity for a particle system:

gameObject.particleEmitter.localVelocity = new Vector3(10, 0, 0);

Also see the documentation