Help with UDF's

I am a new user to MFix, i tried to run some of the tutorials, and getting used to the GUI. I got to know that there are dome things that require an user to write UDF’s.
I am working with DEM, i have a cylindrical arrangement with packed bed of biomass at the bottom end. The biomass is designed as spherical particles initially with the same size. When the fluid flows over the bed, I wish to see an increase in particle sizes.
Can anyone help me write an UDF for this.?