Setting a relative way-point and auto takeoff

Hello, I’m working on an Octocopter with a Pixhawk 1 on it. I want it to be able to auto takeoff after a button is pushed on the drone (no transmitter). After it takes off I want it to fly up to 400’ and then hold there. My questions are:

Is it possible to set relative waypoints, eg: every time it goes to auto mode it goes to 400’ wherever it is and not 400’ in a set lat and long. I do not want to have a ground control or a radio when the drone is operating, so I was thinking about reprogramming some of the firmware to get it to listen for a button push on the I2C port. Does anyone know where I should start with this? Any help is appreciated. Happy Holidays.