All Pakistani pilots flying foreign registered aircraft either convert their existing Pakistani licenses to that country's licence after passing the requisite exams or in some cases their Pakistani license is validated and they can fly foreign registered aircraft. This is exactly what AirSial plans to do with Sri Lankan pilots.All pilots flying in different countries registered aircraft have their licences endorsed by their reverent aviation authority
The point here is that there is no restriction on Pakistani licensed holder pilots to fly foreign registered aircraft; the restriction is on Pakistani airlines.