How can I tie my suitcases on top of my van. It already has a luggage carrier on top. Please help?

I have a Toyota sienna with a roof carrier. I have to go to the airport to pick up my parents next month.. But me , my husband and kids make 5. So if my parents sit all the 7 seats will be occupied. They'll most likely have 4 suitcases and two hand bags. So I wanted to know if I can tie up some bags on top of my car. If yes How ?