No, ArcGIS Online does not support Universal Naming Convention (UNC) paths as hyperlinks. It is not possible to hyperlink to UNC paths in an ArcGIS Online pop-up window.
Note: As an alternative, use URLs (http://) in the ArcGIS Online pop-up window as hyperlinks. There are two ways to configure URLs in an ArcGIS Online pop-up window: • Create a folder in the 'wwwroot' folder, and configure Directory Browsing with Internet Information Services (IIS) Manager. Refer to the solution provided in the following article, Problem: Unable to access hyperlinks in ArcGIS Viewer for Silverlight application pop-up, for more information. • Use Custom Attribute Display when configuring pop-ups. Refer to ArcGIS Blog: Pop-ups: text element essentials .