import .motn files

Apparently not... but you can work around it.

Open the project you want to import. Select the Library tab and the Favorites category. I recommend creating a new folder/theme: there's a small folder icon at the bottom left corner of the Motion window, click on that to add an Untitled folder in the themes section. Double click on the folder name and give it a new name. While the folder is selected, Select all the layers (including Cameras and Lights if you want to include them) and click-drag all together to the Favorites pane and hold until the popup menu asks you whether to save in "Multiple files" or "All in one file". Choose all in one file. Click on the Untitled label and rename it.

Switch to your other project and from the Favorites, drag the saved file into your project.

Do not expect to be able to drag in things like Effect Source/Title Background/Transition placeholders, etc... if your project is a different type... or at all (I've never really tried it.)

HTH