Anything you do with that is company property. Or, rather potentially their property. Courts usually side with employees, but that's a costly court battle.
However wherever possible, I would create the software needed, then put it up for license by the company in the hopes that others would licence it too, so I can make some money on the side. Of course, generalizing it so it wasn't too targeted and generalizing so it did not run afoul of the IPA.