You won't be able to share these folders that way through a firewall - and if you were - it woud be a huge security issue.
You do have some choices though - you can pulish these folders in IIS, install and configure FTP, and use the company web (SharePoint). FTP will be the easiest & least secure - then a webdav folder published in IIS and best & most secure - SharePoint. Both webdav and Sharepoint can use port 80 - so you donlt have to open additional ports.
Here's an aricle about webdav
http://www.windowsnetworking.com/articles_tutorials/WebDAV-IIS.html