You won't be able to open version 9 projects in version 11. Not without some buggy behavior.
But, if you finish your project in version 9, you can output the finished video and use it in a version 11 project.
How you do that depends on whether you're working in standard definition, widescreen or high-def.
Although I do not make it a habit, I have not run into any problems taking a Premiere Elements 9.0/9.0.1 project into Premiere Element 11 (Windows).
What have you encountered in the way of buggy behavior when you have done this?