Currently I don’t believe there are plans to support backing up other sources.
This is logical because backing up remote files causes terrible performance since you essentially download your entire Google drive and re-upload it. And since Duplicati uses hashing for many of it’s comparisons (any time it’s unsure if a file has changed) it will waste a ton of time downloading files.
You can get away with backing up network shares by mounting them into the filesystem and similarly if you get something like Fuse for Google drive you can actually do what you want without Duplicati knowing. But performance will be lackluster and I honestly think time is spent better working on other parts of Duplicati 
Some Fuse Google Drive solutions:
GitHub - astrada/google-drive-ocamlfuse: FUSE filesystem over Google Drive
GitHub - dsoprea/GDriveFS: An innovative FUSE wrapper for Google Drive.