df <- data.frame(treatment = c(rep(c("Stimulation", "Control"), 8)),
                 group = c(rep(c("A", "B"), 4),
                           rep(c("B", "A"), 4)),
                 value = rnorm(16, mean = 150, sd = 20),
                 units = rep(c("u/min", "lit"), 8))


re <- list(A = list(Stimulation = "297.00 ± 19.86", Control = "304.44 ± 14.96"),
           B = list(Stimulation = "4.51 ± 0.07", Control = "4.55 ± 0.10"))

add_unit <- function(list, u){
  z <- lapply(list, function(x){
    list(
      Stimulation = paste(x$Stimulation, unique(u)),
      Control = paste(x$Control, unique(u)))
  })
}

a <- add_unit(re, u = df$units)