Yes, you can!
The trip planner will set the starting point as your current location, as best as we can detect from your device, by default. If your starting location is incorrect or you want to begin a trip from another starting point, it can be easily removed within the Trip Planner. Once deleted, simply add your next waypoint to your trip and drag/drop the waypoints as needed.
You can find some additional info about how to delete and re-order waypoints here: Remove & Rearrange Waypoints