How to make large particles with the use of small particles?

Hello
I saw this simulation on YouTube in which he made a large particle using small particles and modeled the system with the MFIX software.

This is due to the large size difference between the large particle and the small particles inside the system. Because the meshes that are suitable for small particles do not hold a large particle inside.
My question is, how can this be done in MFIX and make large particles with the use of small particles?

The video link is here: https://www.youtube.com/watch?v=y_WBS85nblE

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Make a large sphere with smaller ones, e.g. in a settled packed bed, define all the paticle in a spherical volume as a glued sphere.
Give them a paticle type with des_usr_var at the start of simultion in usr0.f and in usr1_des.f, make them don’t move in x and z directions, only update the force, acceleration and then position of y direction. Of course, the force on the larger one should be the summation of the glued smaller ones.