“外加状语”就是“分离性状语”，也称为“评注性状语”。它不同于“附加状语(修饰性状语)”，不是用来修饰谓语或谓语动词，而是对整个句子加以评论、进行说明或解释、表明说话者的看法或态度。如：

In my opinion, the decision was wholly justified. 我认为，这个决定完全合理。

I'd rather get it out the way, to be honest. 老实说，我情愿把它处理掉。

As far as I'm concerned, the officials incited the fight. 在我看来，是官员们煽动了这场争斗。

Obviously, they don't want to spend too much money. 很明显，他们不想花太多的钱。

Undoubtedly, political and economic factors have played their part. 毫无疑问，政治和经济因素在其中起了作用。

Generally speaking, the coldest weather comes in January. 一般来说, 一月的天气是最冷的。

继续回答：

这种状语不修饰句中的任何成分，而是着眼于整个句子。通俗地说，它有点像“外挂形式”。