You can also use the case_when function within a dplyr pipeline. (You can check the documentation here)

library(tidyverse)

test <- test %>%
    mutate (
        extravar1 = case_when ( 
             YEAR == 2018 ~ var1 + var2,
             YEAR == 2019 ~ var4 + var5),
        extravar2 = case_when (
             YEAR == 2018 ~ var2 + var3,
             YEAR == 2019 ~ var5 + var6))