Radmin VPN for Linux is not planned.

You can hook your NAS to remote PC by using portproxy of netsh.
Example of using it:

netsh interface portproxy add v4tov4 listenport=40545 listenaddress=26.168.0.173 connectport=21 connectaddress=192.168.156.89

listenport - port witch to listen
listenaddress - adress of RadminVPN adapter
connectport - FTP port of your NAS
connectaddress - address of your NAS

And from remote PC you can just add this NAS by using 26.168.0.173:40545

Hope this helps.