Thanks for the reply. 
Let's take the data.frame you created as an example:
sample_df <- data.frame(
year = c(2020, 2020),
month = c(07, 08),
day = c(22, 23),
hour = c(10, 11),
minute = c(01, 02),
second = c(30, 32)
)
In this data.frame, I now have to create a new additional column, which takes the information from the other six columns (year, month, day, hour, minute, second) and provides it in the following format:
2020-07-22 10:01:30