The Design Manager File menu contains a Project Archive command. You can use this command to create a zipped file containing the top-level design file and the entire xproj directory. You can also add additional directories and files to the zipped file.
The Archive Project feature does not allow for a corresponding "Restore Project". An explanation as to why this was not implemented as well suggestions as to how to go about currently un-archiving your project are documented here.
The reason a "Restore" feature was not added to Archive Project in 3.1i is because of the fact that Archive Project allows a user to not only archive their entire project but also archive down to the version / revision level if you so choose to.
Un-Archiving a project becomes a challenging task due to the fact that DM requires additional "smarts" to figure out whether or not a user has archived an entire project or just a portion of it (particular ver or rev). Restoring into an already existing project becomes even more challenging.
Ideas on how how to deploy this are being evaluated for a future release.
For now, it is recommended to archive one's entire project, then subsequently unarchive it via the ZIP tool irrespective of the Design Manager. You can then simply open up the project via File->Open Project, which requires you to identify the projects corresponding <project>.xpj file.