No.
In theory you could subtract the added weight from the capacity plate. However that is dangerous. Are you going to change the capacity plate? If not, when you sell the boat, the capacity plate will be incorrect and may cost the new owner his life.
There are lots of ways to add flotation. One of the simplest is expanding foam to fill voids within the flotation boxes. Another easy way is to fill any empty unused space with foam. Like the bow of the boat. Be sure to read up on the types of closed cell foam and its weight carrying capacity. Adding a 48 quart Coleman cooler as a live-well does two things, gives you a live-well and adds flotation value to the boat. Provided the cooler is secured so it can't separate from the boat in an accident. See my 16' Sylvan build thread for other ways to add flotation. Link in my signature.
A 4x8 sheet of plywood weighs roughly 25 pounds per 1/4" in thickness. If you are adding 1 sheet of 1/2" x 4' x 8' plywood, you added 50 pounds of weight, plus hardware (screws) and any new support structure (2x4, or aluminum angle). How much foam does that require?
I believe there is a sticky that will aid in foam calculations.
Here is a website that does a good job of breaking it down.
https://newboatbuilders.com/pages/flotdiag.html
Note the value for plywood. -0.81 This means plywood has a float value. In general wood floats, to a point. The seas, oceans and lakes are filled with old wooden boats that sank. How many trees do you fish under water? How many screw did you use to hold the plywood in place. What did you use to support the plywood so it supports your weight? All those add weight, and must be part of the calculation. Fortunately these are easy to figure out. Put them on a scale. Screws/nail/liquids are sold by the pound. 1pd box, 5 pd box, 1 gallon = weight, etc.
The other aspect of adding weight is how it will change the boat handling characteristics. If you are adding 180 pounds (deck and 1 trolling motor battery) to the bow, will your motor/prop configuration get you on plane? When anchored up, is the boat nose heavy and easier to swamp by waves? Foam won't correct either of those factors. The foam only comes into play when the boat becomes swamped. If the bow plows through the waves you are more likely to get swamped. If the boat sits nose heavy you are more likely to get swamped. See my 16' Sylvan build thread for weight distribution. I had to move my bow mounted trolling motor battery to the stern. Its a group 31 and about 80 pounds. Enough to make my bow low in the water on plane and sit bow heavy while anchored up. Moving it to the back brought everything back into perspective.
Adding a front deck is great, however there are real things that must be considered. Your life my depend on it. Be safe. Fortunately, it should be easy to account for added flotation needs, and weight distribution. It just takes work to figure it out.