Just to add some detail to what John EB indicated, to show another way, and to be prudent, save your project, do a save as and give it another name, then File, Open, select your other project and you'll get the message:
Select "Add new project." It will be added as one or more Movies. Note that your movie will be the last tab. Now you can copy paste everything from one Movie to another and delete the imported Movie(s), as John EB indicated.
Or, select the Movie that is to be the first part, click on the popdown arrow at the right side of the Movie tabs and select "Merge movies" to open the following screen.
Here you have a choice. If you have already imported the other project, then select the first one. If the imported project contains more than one Movie (or your existing project contained more than 1 Movie), open the pop down and select the Movie that you want to append to the selected tab (Movie). OK. This way, you can append one or more Movies in sequence.
The other choice in the above image is for the case when you have exported a Movie from another project and an MVD file is available. Then, you can navigate to and select the Movie (.MVD) file and it will be appended to your current Movie. So, if you have another project that has multiple movies but you will only want to import 1 movie into another project, then export the single movie from that project.
Simple example
I have a Project in which I have created several template endings for different types of projects, with each one in a separate Movie. I export each Movie - Ending type 1.MVD, Ending Type 2. MVD, etc. Then, when I do a real project, I can use the above method to Merge a selected ending to my current project.