Is there a way to add columns to dataframe that is in the form of function

There is no object created in the global env. In that case, we can use transform

transform(dfs(), new = seq_len(d))

Or use

 `[<-`(dfs(), "new", value =  seq_len(d))

If we want to use $, create an object

tmp <- dfs()
tmp$new <- seq_len(d)

Thanks to @Adam for the suggestions. We can use 1:n() to have more cleaner code.

library(dplyr)

dfs <- function() {
  iris
}

dfs() %>%
  {
    mutate(., new = 1:nrow(.))
  } %>%
  head()
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species new
#> 1          5.1         3.5          1.4         0.2  setosa   1
#> 2          4.9         3.0          1.4         0.2  setosa   2
#> 3          4.7         3.2          1.3         0.2  setosa   3
#> 4          4.6         3.1          1.5         0.2  setosa   4
#> 5          5.0         3.6          1.4         0.2  setosa   5
#> 6          5.4         3.9          1.7         0.4  setosa   6

dfs() %>%
  mutate(new = 1:n()) %>%
  head()
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species new
#> 1          5.1         3.5          1.4         0.2  setosa   1
#> 2          4.9         3.0          1.4         0.2  setosa   2
#> 3          4.7         3.2          1.3         0.2  setosa   3
#> 4          4.6         3.1          1.5         0.2  setosa   4
#> 5          5.0         3.6          1.4         0.2  setosa   5
#> 6          5.4         3.9          1.7         0.4  setosa   6

dfs() %>%
  mutate(new = 1:nrow(cur_data())) %>%
  head()
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species new
#> 1          5.1         3.5          1.4         0.2  setosa   1
#> 2          4.9         3.0          1.4         0.2  setosa   2
#> 3          4.7         3.2          1.3         0.2  setosa   3
#> 4          4.6         3.1          1.5         0.2  setosa   4
#> 5          5.0         3.6          1.4         0.2  setosa   5
#> 6          5.4         3.9          1.7         0.4  setosa   6

^{Created on 2022-01-07 by the reprex package (v2.0.1)}

We could use add_column function from tibble package:

library(tibble)               
mtcars %>% 
  add_column(my_new_column = 1:32,
             .after = "cyl")