If you want to add a field to hr.employee, then you have 2 choices:

1. Add a field in menu "Settings -> Technical -> Database Structure -> Models". Therefore your user must have the access right Technical Features. Afterwards you have to add the new field in the desired views. The easiest way to do this is to activate the Developer mode (see here), go to the view you want to change and e.g. choose Edit TreeView in the drop-down box.
2. Create a new Module (see OpenERP Developer documentation)

You should never insert a new field directly in the database!

Update: I found the following question related to this one here