Hi Firefly, I don't think you can. The only option I know of would be to program a macro using Visual Basic. That is certainly possible, but not simple.
What you can do is set a format so it shows as Time instead of a number (eg. 2:45 instead of 2.75). Maybe that is all you really need?
Change E5 for the cell reference in your spreadsheet. If you have columns of minutes, you could sum the total minutes at the bottom, and fill the formula down to show all hours & minutes (subject to a total of 1439 minutes)
I have come up with another formula for more than 1439 minutes but it is very complex