If you want to forward to an external SIP urI, you'll need to create a new extension (dialplan>dialplan manager). Set the condition to destination_number with expression ^somenumber$ where some number is the number to dial inside the PBX to reach the SIP URI.
Set the action to bridge, with the value sofia/external/sip:sipuser@sipdomain where sipuser@sipdomain is the SIP URI.
Hit save when done. Anytime someone dials the destination_number you set above from inside the PBX, they will be forwarded to your SIP URI.
Hope that helped, if you need further clarification please let me know.
