No, this router cannot act as either a VPN server or client. What it can do is to let VPN packets pass through, but nothing more.
Yes, it is true that DD-WRT-flashed routers can act as either OpenVPN servers or clients. The same applies to other router software like OpenWRT and Tomato.
It is also true that your router can be flashed with DD-WRT software.
Still, there is also another possibility: using a pc in your home to act as OpenVPN server or client, and route traffic through it. There are guides to do this all over the place, see for instance here.
The advantage of using a pc, instead of the router, is that it is easier to maintain a double setup, i.e., one where different pcs in the house are simultaneously on the VPN and off the VPN. Also, it is much less work than flashing the router, and much less risky (check out about the dangers of bricking the router).
The pc acting as OpenVPN terminal need not be dedicated to this (you may start or stop the OpenVPN program with these simple commands
sudo service openvpn start
sudo service openvpn stop
): and all you need to do, to enter the VPN, is to use these two commands:
sudo ip route delete default via 192.168.1.1
sudo ip route add default via 192.168.1.15
assuming your router has IP address 192.168.1.1, and the pc acting as OpenVPN server/client has IP address 192.168.1.15.