You can—with some limitations.
The HUB allows you to upload documents directly from your Google Drive or Dropbox. However, any file you upload exists independently in the HUB and loses “live” connection with the source document. So if you upload a document to the HUB and then make changes to that document in Google Drive or Dropbox, those changes won’t be automatically transferred to your HUB document. If you’d like to update documents in the HUB, you’ll need to make your changes to the original document and re-upload it.