Use flow fild, but calculate only UDS
 

Is it possible to solve equations for UDS by UDF without flow calculation
but with flow field from previous FLUENT solution (with scaled mesh)?

you can easily disable the solution of momentum equations by unchecking "Flow" in the menue Solution Controls/Equations. Convective transport of your UDS will be computed based on this "frozen" velocity field.

cheers

Thank you very much. That helped a lot.