How to combine two rows in a data table into a third new row, such that the values in the row are added together in the new row?

Maybe aggregate?

Hi!  I agree aggregate is an elegant solution 
for this, so I continued your example a bit:

dt_count <- '"","STATUS","N"
"1","Resolved",650
"2","Assigned",135
"3","Closed",530
"4","In Progress",56
"5","Pending",75
"6","Cancelled",20'
dt_count <- read.csv(text=dt_count)

dt_count[
  dt_count$STATUS %in%
  c("Resolved", "Closed"),
  "STATUS"] <-
  "Resolved/Closed"
aggregate(
  x=list("N"=dt_count$N),
  by=list("STATUS"=dt_count$STATUS),
  FUN=sum)

Best,
Rasmus

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