To add a scene from another project to the existing project you are editing:

Step 1: Get the Project ID you would like the scene from, as shown in the image below

Step 2: In the Uploads panel:

a] Go to All Uploads

b] Click the < (back) button on the top left

c] Navigate to the All Projects folder

d] Search for the Project ID you took note of initially

e] Select the scene you would like to add to your existing project

And the new scene should be added to the timeline!