help with sed

[Joep L. Blom]

HI,
I have a little conversion problem and I'm not well versed in sed. I 
know there are several sed-wizards on this list so therefore I hope 
somebody can make the solution.
I have a an ASCII table in .csv format.
The table has 24 columns and >28000 rows (it is a 20 days of data of a 
weather-station in 1-minute intervals).
Now the first 5 columns are day,month,year,hour.minute. all values 
separated by comma's.
I would like to convert that to "day-month-year hour:minute:second", 
meaning inserting an extra ":0" after the minute column.
To give an example: the second row is:
1,8,2011,10,0,....,...,..., etc.
I want this to convert to one date-time column with format:
1-8-2011 10:0:0 or eventually one date and one time column. The rest 
must stay as is (i.e. comma separated).
I hope somebody can help me out.
Thanks in advance,
Joep