As a long time ISV that creates added modules to SL for a specific vertical and integrates to SL, let me strong state that you should never write directly into the SL database. Doing so bypasses all the data checks that the various screens use leading to possible corrupted or unconnected data and, further, every time a new version of SL comes out, you run the risk that your code has to be changed to accommodate a database change.
SL provides Transaction Import for this very purpose. It is powerful, quite easy to use and is the only logical route for integrating data from another software system.