annex, append, attach, combine, integrate, join, put together, inf tack on, unite.
add to D INCREASE. add up (to) TOTAL
Broadband: telecoms giant Openreach to add 200 jobs
Brexit worries ‘ add to car insurance costs’
Stockton Globe Theatre: Structural woes add to revamp cost
To add to it this was a man in the prime of his life. It is just so tremendously sad.