Implementation of a time-variable value

Hey,

I got a question about implementating a value in CFDEM Ligghts.

How do I implement a time-variable value, which shall be variable at every time step?

Is this a possible way to realize my question?
Csurface = fix_Csurface --> vector_atom
vector_atom = Csurface
fix_Csurface = NULL

Thanks for your helpful minds :)