Hi,
You can do that with a formula: http://doc.dataiku.com/dss/latest/advanced/formula.html
If your string always has the leading 0, then you can use:
strval(input_column).substring(0, 1) + ":" + strval(input_column).substring(2,3)
Else, you can use:
if (input_column < 1000, "0" + strval(input_column).substring(0, 0) + ":" + strval(input_column).substring(2,3), strval(input_column).substring(0, 1) + ":" + strval(input_column).substring(2,3))
(if num value is below 1000 then add the 0 manually, else use regular formatting)