I want to do this ( with native species) but I want to make sure it is legal.

My first question is "why would you want to?"

If you want to help national parks, I imagine they would love to have your donation so they can apply it wherever they see funding is needed most.

Get hold of your local conservation office or go to the park service and ask, you might be stepping on someone's plans for the park, or they might not want the kind of trees you want to plant. They may say go ahead, or they may have a definite idea of where they should be planted. Whatever you do, Don't do anything without asking first, then you won't have to worry about being arrested for your good deed.