You can measure the area of anything. Not sure what "square footage" means as "foot" is not a typical engineering unit.
What do you mean by "function?"
If you need something more than Analysis - Measure - Area gives, please describe your problem in more details.
You can also simply change the measurement units, instead of changing the model units. (At least, in Creo 2 you can.)
Open the options dialog:
Change the units:
Nothing I like better than an icon that can't possibly represent what a single word would do; but then it makes internationalization easy by not having to do so, except for the tool-tip. Oh, wait ...