Premiere Elements 11 on Windows 7 (assumed 64 bit)
You cannot import one Premiere Elements project file (.prel) into another (.prel).
If you want to transfer Timeline content between two different project files, then you have two choices
a. Export the Timeline content of project 1 and then import that export into project 2 (assuring that you are keeping your exporting/importing settings consistent between the projects.
b. If you are using a Windows computer, you have the opportunity to do a copy/paste insert between the 2 projects on the same computer using ClipMate in association with Premiere Elements. Details can be found ClipMate Clipboard Extender - Professional Clipboard Manager Software for Windows
This has a good tryout period, but it is not a free software, and I present it for informational rather than promotional purposes.
Please consider, and then let us know if you need clarification on the above.