As TRSTACIE said, the pass covers the only the rail fare portion of the travel. However, when you use a rail pass, you still need to make a reservation for the specific train you will travel on. If I am not mistaken, you should be able to upgrade to a sleeper at the time you make the reservation.

In this case, I would recommend going with the pass for the reasons I mentioned above: it's almost the same price (in fact, it's a few dollars cheaper), which protects you against any possible increases in fare on the route and dates you're looking for and also allows you to take other trains without spending any more money. Adding the sleeper to your reservation will cost the same with or without the pass (that is, if it's a $500 upgrade to the rail fare to get the sleeper, then it will be a $500 charge for the sleeper when you make your reservation and give them your pass numbers.