Not yet, unfortunately. It is only possible to create a new project from a file. The Scratch Team has announced that this will be fixed soon, but for the moment it is not possible.

You can also upload an entirely separate project into a current one by accessing File > Load from your computer in the editor and selecting and loading the desired project. Then, the project will keep its statistics and project id yet still change into an updated version. To get the file of your old project, go to your old project, and do the following: "File > Download to your computer.

Note Warning: Your old project will be lost forever if you overwrite your project with a new one.

To avoid overwriting your old project once you have edited it, you can Save As a Copy before uploading the new project.

Another way is to move the scripts and costumes of another into your backpack, and drag them into your project.