1. 根据《麦克米伦高阶英汉双解词典》,added 只能作前置定语,用于名词前面,不能作表语。例如:
Baby food should contain no added sugar or salt. 婴儿食品不应该额外添加糖或盐。(《麦克米伦高阶英汉双解词典》p.24)
2. extra 可以作前置定语,也可以作表语。例如:
We need extra space for guests. 我们需要额外的地方招待客人。
... , and drinks are extra. ...... , 酒水费另计
(《麦克米伦高阶英汉双解词典》p.715)
added和extra都可表示“额外的”,没什么区别,常可换用。如:
These computers have the added [extra] advantage of being cheap. 这些计算机还有个额外的优点,就是非常便宜。
We will need extra [added] funds to meet all the insurance claims. 我们需要更多资金来支付所有保险赔偿要求。
The house is very comfortable, and as an added [extra] bonus, it’s near to buses and trains. 这所房子很舒适,而且还有额外的好处——离汽车站和火车站很近。