As above you cannot make a PR in to an NPR, but to expand a little you never actually stopped playing either race. When you make one or more player races you can flick between them easily in the F2 economics screen for example. As you stated you started a new race but the humans sat and did nothing, this is because you did not tell them to do anything. In the same way that if you played monopoly on your own and then started using a second piece too, the first playing piece is still on the board and technically under your control. It is just waiting for you to take it's turn, the same applies with player races.
If you look at some of the fiction forums you often will see multi race starts and see lots of things such as China bombed Russia while the United states went off to Mars. All these things are only happening as the person making the fiction thread is playing all three races at once and flicking between each of them on each time increment click thereby simulating each race taking turns.
