FCPX: Use Disk Images to archive your work

Managing Your FCP X Events & Projects using Disk Images
Then you can move your projects to your Disk Image in order to archive them:

Using my NAS with Final Cut Pro X

1. Create a local disk image with Disk utility, whatever the size
2. Unmount it
3. Move it to a given directory on the NAS
4. Resize it to the size you want to give it
5. Mount it