Yes that is OK, some people do this.
You cannot advertise that 3rd party plugin anywhere on Envato item pages or profile pages or live demos.
If you want to sell something outside of Envato then it has to be completely outside of Envato. Cannot use their system to advertise your 3rd party products. You’ll be on your own. Basically anyone who views your item on codecanyon will not know the 3rd party plugin is available for purchase.
You’ll most likely make more sales through codecanyon anyway, people can buy items together in a single transaction. Sure you might be able to sell it for more outside of Envato, but you’ll make more by selling more copies for less though Envato. Hope that makes sense and good luck with the project