As far as I know, PreElements does not let you open a project inside a project
You may, of course, add your video files to any project
There are two possibilities for what I think you are seeking...
a. First Project: Selectively export the segment as an appropriate file to the computer hard drive
b. Import that into the Second Project via Add Media/Files and Folders/
To selectively export, set the gray tabs of the WorkArea Bar to span just the segment to be exported. Then the critical step....in the export settings have a check mark next to "Share WorkArea Bar Only".
Do a copy/paste into between projects with a program such as ClipMate. I have not tried it in version 12, but I have in other versions.
As John T. Smith has pointed out, you cannot import a project file (project.prel) into another.
Please check out the above and let us know if you have any questions on this or need clarification on anything that I have written.