I have a quadcopter using a MultiWii(Arduino Mega) controller. I am curious if there is any way to connect it with ROS capable RPI. (That I could add to the quad itself).
Yes! There is something called the Navio+ it runs a Linux kernel on the raspberry pi with the real-time extension, and runs Adrucopter on top of that.
You can also just connect the serial pins directly to the MultiWii and let the rpi do high level control and let the MultiWii do low level control.
Edit: Here is the reference for the MultiWii serial protocol (MSP) it is very powerful allowing you to do everything from sending RC stick commands, to receiving and injecting GPS values.