.prel are project files, and there is no way to directly combine project files... you would need to create a new computer file (as many time as needed from each different project) and them import each new video file into a new project
Premiere Elements 12/12.1 on what computer operating system?
In the absence of further details, I would generalize...
You would export the content of each of the project files (.prel) to a file saved to the computer hard drive. Then each of the exports is imported into the same grand project for final destination.
The formats of these individual exports should be consistent with one another as well as with the project preset and export settings of the grand project which will be used to generate the grand project.
As an aside, have you updated Premiere Elements 12 to the 12.1 Update yet? If not, please do so using an opened project's Help Menu/Update. One of its major perks is that it gets rid of the Text/Style issue of 12 Expert workspace.