I definitely don't recommend altering/extending any delivered mojoPortal mp_ tables, because then you have forked the code and will have difficulty upgrading in the future.
Take a look a the delivered method to add custom user profile properties. Depending on your needs, that may be all you need. If you have to do any specialized actions based on the contents of your custom properties, you can investigate the available event handlers and see if using one of those would meet your needs.