に加えて (にくわえて) means "in addition to", "on top of", or "not only… but also". It is used to express that something extra exists or happens beyond what has already been mentioned. The nuance is cumulative – you are piling one thing on top of another.
This grammar point attaches to nouns or nominalized phrases.
The item before に加えて is presented as something already known or established, and the clause after it introduces an additional element, often emphasizing that the situation is more significant than expected.
に加えて carries a somewhat formal or written tone, making it common in news reports, essays, presentations, and business contexts. In casual conversation, speakers are more likely to use それに or 〜~だけでなく instead.
に加え is a slightly more concise variant commonly seen in written Japanese. It has the same meaning.