Мы можем использовать dplyr
library(dplyr)
df %>%
mutate(Category = sample(3, n(), replace = TRUE))
данные
df <- structure(list(Species = structure(1:5, .Label = c("Species1",
"Species2", "Species3", "Species4", "Species5"), class = "factor")),
class = "data.frame", row.names = c(NA, -5L))