You can supply vectors to add multiple rows.
library(tibble)
df <- tibble(col1 = c("A", "B"), col2 = c(3, 4))
add_row(df, col1 = rep(NA, 4), col2 = rep(NA, 4), .before = 2)
#> # A tibble: 6 x 2
#> col1 col2
#> <chr> <dbl>
#> 1 A 3
#> 2 <NA> NA
#> 3 <NA> NA
#> 4 <NA> NA
#> 5 <NA> NA
#> 6 B 4
Created on 2020-07-01 by the reprex package (v0.3.0)
Just out of curiosity, why do you want to do something like this? There might be a better way of achieving your desired result.