Unfortunately Symbolic Links and shortcuts don’t work within Typora’s file listing.
I found a work around though:
Create a symbolic link to the folder you wish to add in the default Typora markdown directory, this will then show up in the the file listing, it won’t do anything though if you click on it, as it isn’t implemented or supported in Typora yet.
But, if you right click on the icon link in the file list and choose Open in New Window it will load the linked directories contents.
NB. This trick only seems to work, after you first open Typora. So if it fails; close the app and retry.
This is a nice workaround for me as I have a main notes folder and want occasionally to edit HUGO markdown files in my local HUGO web server directory.