Hi,
You can try adding it as a third party SIP phone. Sample config below
configure terminal
voice service voip
allow-connections sip to sip
voice register global
mode cme
source-address X.X.X.X port 5060
max-dn 20
max-pool 5
authenticate register
authenticate realm local
voice register dn 1
number 4444
name abcd
label 4444
voice register pool 1
id mac xxxx.xxxx.xxxx
number 1 dn 1
username cisco password cisco
HTH
Manish