df1 <- tibble::tribble(
  ~time, ~value, ~option,
  "t-3",   100L,     "A",
  "t-2",   110L,     "A",
  "t-1",   120L,     "A",
  "t-3",   270L,     "B",
  "t-2",   459L,     "B",
  "t-1",   373L,     "B"
)
df2 <- tibble::tribble(
  ~start, ~option,
  2020L, "A",
  2025L, "B"
)

library(tidyverse)
library(glue)
full_join(df1, df2) |>
  rowwise() |>
  mutate(
    time = eval(parse(text = glue("function(t){time}")))(start)
  ) |>
  select(-start) |>
  ungroup()