Because EJS is a java code generator, so in principle, you can do whatever can be done with java.

You will need to know the EJS build in function names which can add view elements into EJS _view.

If you want to have a look at some examples, you can check out xxxView.java in your output foloder.

But I never did it that way. EJS let you define variable array.
If I need to add many particles in run time, I will define an array of variables and use view or number of element property to change the number of paricles shown.

I would suggest you check out  [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=864]Charged Particle and Wall [/url]. You can add up to 500 particels and build walls in run time.
I think it is better to do it this way.  ;)