Can I connect files already existing in SharePoint to sFiles?
Absolutely! Simply navigate to the Salesforce record to which you want those files to connect. Click "Set up with SharePoint" button on the sFiles browser and sFiles will create a folder for that record inside SharePoint. Over in SharePoint, drag-and-drop your files that you want associated with that Salesforce record into the new SharePoint folder that was created by sFiles. You will now see the files show up in Salesforce.