@Will gave you the correct solution. Project files are XML files that list which packages are part of a project. You can add an existing package back without any issues. You can even manually add a node if you want by editing the file directly. I use to find this useful before BIDS Helper offered sorting capabilities.
You may also want to implement a version control system if you are working with SSIS. Every once in a blue moon a package gets into a funky, unrecoverable state and we have to rollback to a previous version to get it working again. This happens about 4 times a year for a team of 6 people who work on 100-200 packages. Also, you will never lose a package again even if you erase it on the server and your local copy is wiped out.