This PrE FAQ Entry will give you the instructions for combining multiple Projects: http://forums.adobe.com/thread/1300400?tstart=0
To get that in French, the Translate button (fairly new), should handle most of it.
To sum it up, you cannot import one project file (project.prel) into another to put them together.
There are at least two approaches to consider
a. exporting the Timeline of one to an appropriate file format and then importing that saved file into the second project from which you will actually do the exporting...it is important that you coorindate the file format and the project presets accordingly.
b. you can try to copy/paste insert Timeline content between the project files (project.prels)....the details for doing that are described in the following link.
Premiere Elements 12 and ClipMate results will be the same as for Premiere Elements ClipMate. The shortcoming for 12 and 11 found so far is that transitions are not copied/paste insert as you will read about in the link given above.
If you need any details specific to your situation, please supply more details and we will offer more specific information.