Add a new row based on test set predicted values and time stamps

Hi all,

I have the prediction for my test set which are forecasted Value for "4/1/2020" for each match of "id" and "Group". I would like to add a fourth row to each group by (Group,id) in my train set and the values for this row should come from test set :

my train set:?

? ? ?structure(list(Date = c("1/1/2020", "2/1/2020", "3/1/2020", "1/1/2020",?
? ? ?"2/1/2020", "3/1/2020", "1/1/2020", "2/1/2020", "3/1/2020", ""
? ? ?), Value = c(3.5, 2.7, 4, 2.5, 3.7, 0, 3, 0, 1, NA), Group = c("A",?
? ? "A", "A", "B", "B", "B", "C", "C", "C", ""), id = c(1L, 1L, 1L,?
? ? 101L, 101L, 101L, 100L, 100L, 100L, NA)), class = "data.frame", row.names = c(NA,?
? ? -10L))

test set:

? ? structure(list(Date = c("4/1/2020", "4/1/2020", "4/1/2020", ""
? ? ? ), Value = c(3.5, 2.5, 3, NA), Group = c("A", "B", "C", ""),?
? ? id = c(1L, 101L, 100L, NA), value = c(0.2, 0.7, 0.9, NA)), class = "data.frame", row.names = c(NA,?
? ? ?-4L))structure(list(Date = c("4/1/2020", "4/1/2020", "4/1/2020", ""
? ? ), Value = c(3.5, 2.5, 3, NA), Group = c("A", "B", "C", ""),?
? ? id = c(1L, 101L, 100L, NA)), class = "data.frame", row.names = c(NA,?
? ? -4L))

desired output:?

? ??structure(list(Date = c("1/1/2020", "2/1/2020", "3/1/2020", "4/1/2020",?
? ? "1/1/2020", "2/1/2020", "3/1/2020", "4/1/2020", "1/1/2020", "2/1/2020",?
? ? "3/1/2020", "4/1/2020"), Value = c(3.5, 2.7, 4, 0.2, 2.5, 3.7,?
? ? ?0, 0.7, 3, 0, 1, 0.9), Group = c("A", "A", "A", "A", "B", "B",?
? ? "B", "B", "C", "C", "C", "C"), id = c(1L, 1L, 1L, 1L, 101L, 101L,?
? ? 101L, 101L, 100L, 100L, 100L, 100L)), class = "data.frame", row.names = c(NA,?
? ?-12L))

Data is dummy and I have milions of records in original data set.?

Thanks for any help.
Elahe?
?