Tricky. You need to differentiate between ball, own goal, other goal and other bot.
You also might want to use the line follower to avoid that bots leave the play field.
Maybe mark one goal with a red lamp, the other with a green lamp. Use a forward looking tube with red or green filter and a mirror so the light sensor can find the target goal direction.
Maybe mount a micro switch on forward looking pole which is higher than the ball. If the pole hits something, it is the other bot or a goal. If the object detected by the ultrasonic sensor comes closer, then it is the ball.
You could use the other slot on the rj25 connector for a servo which kicks the ball.