In Premiere Elements you have to export (share) a file to .avi from the original project and then bring the .avi into your other project.
Only in Premiere Pro can you import a project into a project.
You can output a finished video from each of several projects and then join them in a new project.
Some find the H.264 MP4 outputs of sufficient quality that there is no visually noticeable loss from rendering. Others find it better to use a "lossless" codec to output from the sub-projects for the mater project. The lossless Lagarith codec is available as shareware on the internet. It's downside is that the intermediate files you produce will be large.
Thank you very much