You can’t do what you want, but you can edit the trip and manually add a stop at your preferred Supercharger site(s). For each of the automatic charging stops that you don’t like, add a manual stop at the preferred location. If you select a supercharger site as a stop, it will do the normal calculations for how long of a charging stop you need to make there to continue on your trip. On a recent trip, I did this, keeping the next 5 or 6 charging stops along the route as I made my way through the day’s driving.
Select the 3 dots in the window with the distance to your next turn, select Edit Trip, and the add/remove/reorder stops on your route, selecting Done to get new navigation directions.