You can add a link to the current navigation in SharePoint Online via the client side object model with the following method: 
Download the code here: AddNodeToCurrentNavigation
The method was inspired by this source: http://www.c-sharpcorner.com/Blogs/12172/how-to-create-a-node-in-the-quick-launch-bar-using-csom-in-s.aspx
Rik Dekker 