可以使用交叉联接：

setDT(DT)[
  CJ(group = group, val = val, unique = TRUE), 
  on = .(group, val)
]

    group val somethingElse
 1:     1   1             1
 2:     1   2             1
 3:     1   3            NA
 4:     1   4             1
 5:     2   1            NA
 6:     2   2             1
 7:     2   3             1
 8:     2   4            NA
 9:     3   1             1
10:     3   2             1
11:     3   3             1
12:     3   4             1

我只想为稍微复杂一点的情况加上这个答案：

#Raw Data
DT = data.table(group = c(1,1,2,2,2,3,3,3,3),
                  x = c(1,2,1,3,4,1,2,3,4),
                  y = c(2,4,2,6,8,2,4,6,8),
                  somethingElse = rep(1,9))

#allowed combinations of x and y
DTxy = data.table(x = c(1,2,3,4), y = c(2,4,6,8))

在这里，我想将DTxy中的所有x、y组合添加到DT中的每个组（如果还没有）。
我写了一个函数来处理子集。

#function to join subsets on two columns (here: x,y)
DTxyJoin = function(.SD, xy){
  .SD = .SD[xy, on = .(x,y)]
  return(.SD)
}

然后，我将该函数应用于每个组：

#add x and y to each group if missing
DTres = DT[, DTxyJoin(.SD, DTxy), by = c("group")]

结果：

group x y somethingElse
 1:     1 1 2             1
 2:     1 2 4             1
 3:     1 3 6            NA
 4:     1 4 8            NA
 5:     2 1 2             1
 6:     2 2 4            NA
 7:     2 3 6             1
 8:     2 4 8             1
 9:     3 1 2             1
10:     3 2 4             1
11:     3 3 6             1
12:     3 4 8             1