Is it permissible to combine prayers for the sake of work in a foreign country where it is not allowed to perform prayers at their prescribed times?

If by "I combine all prayers" you mean performing all five prayers at one time, then this is not permissible. The legally sanctioned combining of prayers is between Dhuhr and Asr, performed during the time of either of them, and between Maghrib and Isha, performed during the time of either of them. As for combining all five prayers, this has not been legislated.