Hi,
There is no "feature" in mojoPortal which allows you to edit source code. You can build your own features that plug into mojoPortal.
Documentation on building custom features begins here:
http://www.mojoportal.com/addingfeatures.aspx
Sounds like you would prefer the inline code approach. I recently added this page to the documentation which may help you.
http://www.mojoportal.com/usinginlinecode.aspx
Best,
Joe