Automatic way to offset surfaces

No automatic way I know of.  We just put a note on there specifying that the dimensions apply to finished (anodized typically) surfaces unless otherwise specified.  Since your vendor is far more familiar with the thickness of the coatings they apply than you, I'd send them the finished part STEP files and let THEM figure out the offsets needed.  If they whine about it tell them that there's other vendors out there you can do business with.

Creo supports application of a shrink feature in the mold design extension (add on module). I am not sure that is what you are asking for here though.

If you can define mathematically a nominal value for the surface offset, then you can include this design intent into your design model. If you can clarify in more detail what you need to do with a simple example it would help with proposed solutions.

Assuming you determine that you need 2 or more dimensional variants of a design model available as Creo parts, then the Inheritance feature would support this directly. You would have at the top level the "as finished design model" model and then use the Merge/Inheritance function to create derivative models representing the different geometries associated with the manufacturing processes.

When done intelligently this would allow you to modify the top-level model and the derivative models would update automatically with the required offset dimensional changes. You should be able to address the tolerance assignment using annotation elements.

For prismatic geometries this should be quite simple to manage. If you have more complex form factors on parts, then offsetting them may require some manual intervention in Creo.

I've always had trouble with trying to specify the "before finish/coating/plating/etc" dimensions because different vendors or processes have always needed different #'s based on their equipment. It seems to create a never ending do-loop of "required changes" since there is a never ending set of processes/vendors/machines. Unless forced, my go to has always been to only supply the finished dimensions...of course that never works for internally supplied parts but that is usually a more controlled setting.

No automatic way that I have seen.  You can copy all surfaces (select a surface, hold shift and select surface again, hold ctrl and select any non-finished surfaces if needed, ctrl+c, ctrl+v) offset quilt, remove material with solidify.  If there are any unfinished surfaces you may need to extend the quilt to fully intersect the model.