Shelf gives you a number of options when it comes to syncing and/or importing content from your cloud accounts. Options include:
Adding a single file
Adding multiple files at once
Adding a single folder (and all of the files and subfolders underneath it)
Adding multiple folders at once
Adding a mix of files and folders at the same time
For details on how to perform a sync or one time import, please refer the question "What are the different ways I can connect Shelf to content that's in another cloud account?"