If you know the weight of the part when powder coated you can overide the weight of the part manually, see this thread https://forum.solidworks.com/thread/88903?q=weight%20of%20part.
Another way to do this is to create a custom material with the weight of the powder coat included. If as you say your parts increase by 10% you can create a custom material with the weight increased by this amount then use this material as the material for your powder coated part. Lets say your parts are from plane carbon steel, you could create custom materials of plane carbon steel +1% another +2% and so on so as you can adjust the weight as necessary depending on how much is powder coated.
Nice refference, Thank you
For parts that my company selectively plates, I will use a split line (if necessary to break up an otherwise uninterrupted surface) and then use the offset surface command to the thickness of the plating and finally the thicken command back in the direction of the original surface (being sure to select the option to not merge the two bodies). This leaves you with two bodies and you can assign the appropriate material to each body to get total mass. As a note, I'm making comparatively small assemblies so the extra geometry isn't much of a drain. This may not be good to use if your parts are going into a larger assembly.
I've got an example of something we do here that has the above mentioned features but can't seem to find anywhere to attach it. Let me know if your interested in it.
I created the attached macro for handling this. It has default values for Nickel, Gold and Zinc. You can also put in values for your own coatings. Just enter the thickness and the mass in pounds per cubic inch. If the entire part is not plated enter the masked area in square inches. It works with two platings, we often do gold over nickel. The macro will create custom properties for the total mass as well as the mass of each coating.
If you change the part geometry you'll need to rerun the macro to update the masses.
This should give more accurate results than creating a custom material. The macro works with the surface area calculated in the mass properties tab.