Hi,
You should not fork the code or you will not be able to upgrade so you should not just start changing code in mojoPortal
http://www.mojoportal.com/avoid-forking-the-code.aspx
You have 2 viable approaches that are possible.
1. You could buy my add on product Form Wizard Pro which makes it easy to create custom forms.
2. You can clone the contact form feature into your own custom feature and then modify it, but this requires development expertise.
Best,
Joe