Can we use OpenSTA as a functional or regression testing tool.
Can we add GUI , Database , Bitmap cheackpoint ?
you can (to some degree) but I wouldn't, you'd be better off using a functional testing tool.
In OpenSTA you can view the HTML of the response so you can always check it against what you know to be a correct response.
The right thing to do though is to use the right tool for the job (i.e. a functional testing tool).
Everywhere's within walking distance if you have enough time.