Sentence:  His life-long friend , Serge Koussevitzky , gave unreservedly of his praise and brilliant performances in Boston , New York , and Washington , D. C. , to which he <WSD>added</WSD> broadcastings and recordings for the whole nation . 
Meaning of the word marked with <WSD> tags:  make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of