What version of Premiere Elements, what computer operating system?
You cannot import one Premiere Elements project file into another Premiere Elements project file.
If Windows, you have at least 2 choices....
1. Export the Timeline content of project file 1 to a file saved to the computer hard drive. Then import that export into project file 2 using the project file 2's Add Media or Get Media/Files and Folders. Keep the import and export settings consistent from project file to project file. Some suggest using a Lagarith.avi file as the intermediate in this case. Other considerations possible, to be discussed if necessary.
2. You could explore the copy/paste insert between parts using ClipMate in conjunction with Premiere Elements. ClipMate is a Windows only program.
Please consider and supply details. If any questions, please do not hesitate to ask.