You can use any third-party cloud service to sync and backup your WriteMap files and exports.
With WriteMapper, your work gets saved as regular files on your device's filesystem, as WriteMaps (.writemap format file documents). These files can be moved, copied and synced across your file folders, just like any other of your files you're used to working with.
As such, you can also sync these files across your devices using your preferred cloud syncing service. WriteMapper doesn't force you to use its own syncing service, so that you can manage your files in your local file system however you like, without being bound by third party cloud storage.
If you wish to sync your WriteMapper-related files across devices, you can do so without restrictions using the interface of any third-party cloud sync service such as iCloud Drive, Dropbox, Microsoft OneDrive or Box.