If you want to add a mass source term, there is a derivative of the mass fraction in
 

In the component transport problem, in addition to the convection term, the component conservation equation also contains a coefficient multiplied by the derivative of the mass fraction in the x direction. Can this item be regarded as a mass source term ? How to write this udf if you can ?
Is there any other way to deal with it ? This is a convection diffusion problem. The governing equations are mass conservation and momentum conservation.
Thank you all

The coefficient multiplied by derivative is the diffusion term.