I would recommend using a plugin to store and retrieve custom content from the database: http://www.jivesoftware.com/docs/jive_sbs/4.0/index.jsp?topic=/com.jivesoftware.help.sbs.online_4.0.8/developer/AccessingtheDatabasefromaPlugin.html
If you would like to store custom content into the database I would not add additional columns to an existing table, but instead create a new table to store your custom content. That way future upgrades will be easier to perform and there is less of a risk of decreasing the performance of an existing table.
Thanks for the information Curtis.
I do agree when you say it is not suggested to make changes in the existing tables so as to support future upgrades.