Nope, you've used up your trial. Adding a service to an account doesn't give you another free trial. It's considered the same account, so it's adding services, just like if you went from one DVD to three, and so on. The new charges apply right away.
Now had Netflix gone ahead with their divesting the DVD service into the ill-fated Qwikster project, that might have been possible at some point.
Don't believe everything on the Interwebz! A duck's quack DOES echo!