Well, if I understand you correctly. You can do something like the following.
To show it, I first create a data.frame with your example
df <-
scan(what = character(), sep = ",", text =
"001, 34, 3, aa.com
002, 4, 4, aa.com
034, 3, 3, aa.com
001, 12, 4, bb.com
002, 1, 3, bb.com
034, 2, 2, cc.com")
df <- as.data.frame(matrix(df, 6, 4, byrow = TRUE))
colnames(df) <- c("user_id", "number_of_logins", "number_of_images", "web")
You can then run one of the following lines to add a column (at the end of the data.frame) with the row number as the generated user id. The second lines simply adds leading zeros.
df$generated_uid <- 1:nrow(df)
df$generated_uid2 <- sprintf("%03d", 1:nrow(df))
If you absolutely want the generated user id to be the first column, you can add the column like so:
df <- cbind("generated_uid3" = sprintf("%03d", 1:nrow(df)), df)
or simply rearrage the columns.