I would like to be able to click a link in a popup and it open a folder on our local network. Is this possible?
You could Using Hyperlinks—Help | ArcGIS for Desktop if you have a field in your table with the appropriate paths.
Can this be configured in WAB for Portal?
Are your network items stored on a server that can be accessed by the web? If so, then I would think so. Here are some ways to share items in ArcGIS Online (which works with WAB):
Share items—ArcGIS Online Help | ArcGIS
Actually, you mentioned Portal. Take a look at this. This may be more what you need to share items regarding your portal account:
Share items—Portal for ArcGIS | ArcGIS for Server
Yes these are network items that can be accessed. Can the File location be used in the URL field for a popup in WAB?
I do not think this is possible since the web is trying to access a local network drive that is not linked to the web.
Your best bet is to try looking at the options from Share Items in Portal:
Retrieving data ...