Hi,
I don't think you can do it on a formula, but you can do it on text by going into edit mode, selecting the text to have the format changed, and then making the changes via the formatting toolbar, which you will find is active. If you try this on part of a formula you will find the formatting options greyed out.
What you really need is a Text function which could be inserted as part of the formula, or possibly a User Defined Function to do what you want (If it can be done via a UDF).
Good Luck,
Peter Moran