Yes, that can be done.
Menu, System Settings, Sip Proxy.
Choose a spare Proxy, eg Proxy 1)
Enabled: YES
Display Name: [A display name]
Username: [Your VoIP Stunt Username]
Authorization User: [Your VoIP Stunt Username]
Password: [Your VoIP Stunt Password]
Domain/Realm: sip.voipstunt.com
SIP Proxy: sip.voipstunt.com
Out Bound Proxy: sip.voipstunt.com
To dial using VoIP Stunt either prefix the number with a # and then the proxy number eg) #1 or right click and to bring up a menu then select the VoIP Stunt account. You will have to dial in the international format eg) 0044121................