Let's start here. The UC520 is for VOIP. It is not a firewall, and as such is not intended to act as a VPN gateway.
That said, your question "is it possible". In theory it is possible if you can obtain the necessary secure IOS software package and load onto the router. This will open up the command set you need to configure VPN tunnels. Would I try? Not in a million years (you want your phone communication to remain as stable as possible, right?).
If you have a firewall, use it for your VPN. If you do not already have a firewall, I recommend getting one. You are already a Cisco shop, so a Cisco ASA might be your first choice. I am not terribly keen on Cisco firewalls though, and would recommend looking at the Juniper Netscreen. On the lower-end, 3COM makes a deacent OfficeConnect series that supports SSL and PPTP VPNs. SnapGear also seems to make a fairly deacent low-end (cheap) unit.
