Hi Jacques,
Yes, if you are a developer you can create your own tables and features to interact with data in your tables. Most custom features do involve creating custom tables and we have an installation system that allows you to add sql scripts to create your tables stored procedures etc and alter them over time through version upgrades.
I recommend watch and work through the training videos which shows how to do the most common tasks for any feature including creating a table and data access code. That should get you oriented to how to develop custom features, after that the next step is to realize the the mojoPortal source code has examples for how to do many many things that you are likely to want to do in your own features and studying the code for existing features will help you a lot in figuring out how to do what you want in your custom features. While we have a lot of documentation on this site, the source code itself is really the ultimate documentation and it is written with the goal that it should be fairly easy to understand the code.
http://www.mojoportal.com/developertrainingvideos.aspx
It is a lot of videos but each one is less than 10 minutes and a lot of information is covered.
Hope it helps,
Joe