I have a D-Link DGL-4500 router and it has a USB port on the back is their anyway to connect a USB hard drive and access the files Via the network without using a VPN.
Also, i have not been able to track down any custom firmware that would allow you to do it (and your router is not supported by openWRT) so i think its a dead end just now (but that might change in the future).
What sort of files are you wanting to share? I / We might be able to suggest another solution