Not sure you can do this with PJSIP. You can create a block of dialplan to do it, then change the trunk context to match:
[from-pstn-nunogato]
exten => _.,1,Noop(Entering user defined context [from-pstn-nunogato] in extensions_custom.conf)
exten => _.,n,goto(from-pstn,xxxxxxxxx,1) ; substittue DID for x's