Joining salaahs when travelling

Q: Is it permissible to join the salaahs when a person undertakes a journey and is a musaafir? 

For example, you are leaving a country and it is zuhr time, so before the flight, you join zuhr and asr. Or, you visited a country and you go to a shop and maghrib time sets in, so you read maghrib and esha together. Is this permissible, and if not, does one get a sin when one does it?